A little-known section of the Pension Protection Act of 2006 could put a halt to donations of extremely valuable works like Henri Matisse’s “Plum Blossoms,” valued at $25 million, which was given to the Museum of Modern Art last year by its president, Marie-Josée Kravis, and her husband, Henry Kravis.

The law makes changes to a mechanism known as fractional gifts, which museum directors say is crucial to their donations. In a fractional-gift donation, the donor might, say, give 10% of a work in the first year, 10% the next year, and the remaining 80% either over the course of his life or upon his death. From the donor’s perspective, the advantage is twofold: First, he can stretch out the tax deduction, which is important if the work is very valuable. Second, he makes a commitment to the museum but can, if he wishes, maintain part-time possession of the work during his lifetime.

The Pension Act makes two changes that, museums say, put severe restrictions on fractional gifts: It limits the total period of the donation to 10 years. If the gift isn’t completed either within 10 years or at the time of the donor’s death — whichever happens first –– the Internal Revenue Service will recapture all the previous deductions, plus interest.

The law also effectively diminishes the value of the artwork, as it affects the deduction the donor can take. Under the past system, the donor got a new appraisal at each point and deducted the appropriate percentage of the current estimated value of the work. Under the new law, the donor still gets a fresh appraisal, but then he has to take his deduction based on the lower of the two appraisals. In other words, if a $10 million painting appreciates to $15 million, the donor giving 10% of it can only take a $1 million deduction. But if the work depreciates to, say, $9 million, then he can only take a $900,000 deduction.

This may be a calculation remote from most people’s lives, but museum directors say they depend on this intricate system of financial incentives to stimulate people’s generosity and attract works that the museum could never afford to buy. If the balance between the advantages of donating versus selling shifts, wealthy individuals will be much less likely to give a valuable painting or sculpture away.

Under the new system, “any tax adviser would say to somebody with that kind of wealth, ‘You’re insane to do this. Put it in your will,'” the director of government affairs at the Association of Art Museum Directors, Anita Difanis, said. The problem in that case, she explained, is that the artwork may never get into the will: “The kids suddenly want it, or some dealer makes an offer they can’t refuse.”

The director of MoMA, Glenn Lowry, said he expected the new law, if left unchanged, will lead to a sharp drop in donations. “In a system like ours that depends on private philanthropy, you have to have the most advantageous mechanism possible in order to ensure that people will give to you rather than sell on the open market,” Mr. Lowry said.

“By making it less advantageous to give, many people simply won’t choose to do so –– or won’t even be able to afford to do so,” he added. “Even for people of great wealth, when they’re giving away works of art that are valued at tens of millions of dollars, if not more, they represent a very substantial portion of those people’s assets.”

The senior counsel for the Senate Finance Committee, Dean Zerbe, said the old system was rife with abuse. “Very wealthy people were taking huge deductions, and keeping the art at their homes,” he said. “It wasn’t going to the public benefit, or only in many, many years.” He called the changes “a pretty commonsense way of dealing with it.”

But both Ms. Difanis and Mr. Lowry said, as far as they are aware, there is little, if any, abuse. The reform “is a reform in search of a problem,” Ms. Difanis said. She attributed it to “paranoia” that people are scamming the system. “They have one or two examples” of donors not making good on their gifts, she said, “and extrapolate from that that this is what’s going on everywhere.”

The Association of American Art Museum Directors is lobbying the Senate Finance Committee to change the law, and hoping to reach a compromise, Ms. Difanis said. In her view, the standoff between the legislators and the museums represents something of a clash of cultures. “The lawyers and accountants who are this committee staff like to have things line up: If you take a deduction, the public gets an immediate benefit,” she said. “We don’t line up, so that’s hard on them.”

In the meantime, she’s trying to bridge the gap. “We’re trying to say, ‘Well, the public actually does get an immediate benefit from that first 10%,'” she said. “It means the public has the right to ownership to that very valuable piece of art. It can’t go anywhere else. In the end, it goes to the museum. So that first 10% is very valuable.”