The photoaddition of p-acetylstyrene (PAS) triplet to styrene (STY) has been studied. Direct irradiation of PAS in the presence of styrene yielded trans-1-(4-acetylphenyl)-2-phenylcyclobutane, an expected addition product, its cis isomer, and 1-(4-acetylphenyl)-1,2,3,4-tetrahydronaphthalene. The rate of addition of (3)PAS to styrene was determined by laser flash photolysis to be 3.2 X 10(6) M(-1) s(-1), and by quantum yield measurements as 2.0 X 10(6) M(-1) s(-1). The rate is 1 order of magnitude greater than that of addition of a primary alkyl radical to styrene and thus in only fair agreement with the 1,2-biradical model. The reaction of 3PAS with p-methoxystyrene is even faster. A rough estimate of the rate constant for (STY)-S-3 + STY is in better agreement with expectation. The possible involvement of a weak exciplex between the perpendicular PAS triplet and the styrene ground state is suggested to rationalize the rate enhancement. The validity of the 1,2-biradical representation of alkene triplets is discussed.