An analytical formula for the description of exchange-relaxation mechanism of spin catalysis is derived. The formula is applied to describe the experiment on recombination of a radical pair obtained by diphenylpenthanon (DPP) photolysis on addition of spin catalyst. 2,2,6,6-Tetramethylpiperedin-1-oxyl (TEMPO) was used as a catalyst [4]. Good agreement between theory and experiment is shown. (C) 2011 Elsevier B.V. All rights reserved.