U.S. states' tax revenue growth probably peaked earlier this year as preliminary figures for the second half of 2013 have softened significantly, a study released on Monday said.

State tax revenues rose 9 percent in the second quarter from a year before, according to the latest State Revenue Report from the Nelson A. Rockefeller Institute of Government.

Early third-quarter figures from 47 states, however, suggested that revenue gains have slowed to 6.1 percent from a year earlier, according to the Albany, New York-based group, the public policy research arm of the State University of New York. Personal income tax collections grew by 5.3 percent and sales tax collections increased by 5.6 percent.

Gains in the first half of the year were led by Western states, with the Far West notching a second-quarter revenue gain of 14.9 percent, the report said. Much of that was attributed to California's increase of 21.2 percent.