For the last century, art collections have generally flowed from culturerich Europe to cash-rich America. Now, there are signs of them going the other way. Works from three major American collections of Impressionist and modern art will go on the block next month at Sotheby’s in London — a result, art world insiders said, of the increasingly worldwide art market and also the rise of the pound relative to the dollar.

“I think there are two reasons” that this material is being sold in London, the director of Impressionist and modern art at Sotheby’s in London, Philip Hook, said.

“One is that the pound is very strong, so it’s exciting for American vendors to be selling in pounds. And the market is exceptionally strong, so people want to sell at the first possible opportunity. The next sale happens to be the London sale. It’s a reflection of the true globalization of the market.”

The sale, on February 5, includes three major paintings — a Renoir, a Vlaminck, and a Dufy — from the collection of the late Charles Lachman, a founder of Revlon cosmetics. Lachman acquired all three works in the 1950s, so they haven’t been on the market in half a century. The Renoir, “Les Deux Soeurs,” is estimated at six to eight million pounds, or $11,830,000–15,770,000. According to his widow, Lachman provided much of the initial $300 with which he and his partners, Charles and Joseph Revson, founded Revlon in 1932. As a chemist, he also mixed the ingredients for the original Revlon nail polish.

The other two collections being sold at Sotheby’s are those acquired by Paul and Mary Haas of Corpus Christi, Texas, and by the late New York lawyer Herbert Singer and his wife Nell. The Haas collection includes painting by Cézanne, Monet, Sisley, and Caillebotte. The Singer collection is particularly strong in modern sculpture, including works by Jacques Lipchitz and Jean Arp, as well as, Mr. Hook said, “a beautiful stone head by Modigliani, which is a great rarity.”

It’s difficult to say which might have been the most important factor — the timing of the sale, the strength of the pound, or the field of potential buyers — in these collections ending up for sale in London instead of New York. “Sometimes time is the issue,” the dealer Dominique Levy, of L&M Arts, said. “You’ve missed the November sales here, and you want to take the opportunity of this very strong market, and you don’t feel safe waiting.”

“We tend to recommend, if people want to sell, that they sell in the next available major sale, and it just depends whether it’s London or New York,” Mr. Hook said.

Yet Ms. Levy and others also emphasized that what you’re selling still makes a difference in deciding where you should sell it.

“I still believe it is a strategic decision where to sell,” Ms. Levy said. “It all depends on what the focus of the collection is and which buyers the auction house is targeting.” The strength of the pound and the euro, she added, has made Europe a bigger buying territory than it was. “London has grown in the last two years in an absolutely tremendous way,” she said. (The numbers back her up: Sotheby’s series of Impressionist and modern art sales, which carry a combined estimate of over $200 million, are the most valuable series ever staged by the company in Europe.)

The co-head of postwar and contemporary art at Christie’s, Amy Cappellazzo, said that the interests of buyers in New York and London are still distinct, and that can influence where a work ends up for sale. “It’s no question New York is a larger market,” she said. “But there are certain examples or types of work that fly better with the European buyer. There’s a different idea of what’s a pretty picture in Europe versus what’s a pretty picture in America.”

Nonetheless, both Christie’s and Sotheby’s do their best to drum up demand by flying works all over the world prior to a sale. Forty-eight works from the Sotheby’s Impressionist and modern sale will come to Sotheby’s in New York, where they will be on exhibit to the public from this coming Sunday to Tuesday. After that, they will travel to Palm Beach, Fla., and Paris and back to London. In the meantime, German and Austrian works from the sale will be exhibited in Germany, Austria, and Switzerland, before also going to Paris.

In addition, the most important buyers get house calls. “Something we do increasingly is travel individual pictures to individual prospective buyers, if the pictures and the buyers are important enough,” Mr. Hook said. “That sometimes pays dividends — for the prospective buyer to have the opportunity to hang the picture on his wall prior to the sale.”

All this traveling means “an incredibly complicated logistical operation” for the auction house, Mr. Hook said, as well as a substantial investment in art transport fees and insurance. Mr. Hook estimated that Sotheby’s spends 1 or 2% of the value of a sale on traveling exhibitions, but he said it pays off in the success of the sale. “We travel things as much as we can, in order to get them in front of as many potential buyers as possible,” he said. “Because that is how you ratchet up competition and therefore prices.”

Exactly how the strength of the pound and the euro will affect sale prices in London is hard to predict. One place the exchange rate did have an impact was at the New York sales in November, where European buyers found themselves wielding new power. “I was on the phone with a few of them, and they regularly were asking me what the bid was — what was it in dollars, and what was it in euros,” Ms. Levy recalled. “And suddenly they were going one or two bids higher than they intended because of the currency strength.”