The time-resolved H-1 photo-CIDNP formed upon photocleavage of diisopropylketone in the presence and absence of stable radical (TEMPO) and dodecanetiol-1 as radical traps has been studied. It was found that TEMPO affects CIDNP formation in a primary radical pair not only as a radical acceptor but also through spin exchange. The rate constant of the recombination of isopropyl radicals and TEMPO in acetonitrile at ambient temperature was determined as (2.4 +/- 0.5) x 10(8) M-1 s(-1). (C) 2002 Published by Elsevier Science B.V..