The addition of the 4-methoxystyrene radical cation to neutral 4-methoxystyrene and the cleavage reactions of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation in acetonitrile have been studied by product analysis and by nanosecond and picosecond transient absorption spectroscopy. The main product upon radical cation mediated dimerization of 4-methoxystyrene using chloranil as the electron transfer sensitizer is a substituted dihydronaphthalene, 1,2-dihydro-7-methoxy-1-(4’-methoxyphenyl)naphthalene, at low concentrations of 4-methoxystyrene. At higher concentrations, the main product is 1,2-bis(4-methoxyphenyl)cyclobutane. Cleavage of the cyclobutane radical cation is found to give 4-methoxystyrene and the dihydronaphthalene in a 1:3 ratio. In the time-resolved experiments, the radical cations are generated from 4-methoxystyrene and 1,2-bis(4-methoxyphenyl)cyclobutane by 266-nm photoionization or by 355-nm photoinduced electron transfer using triplet chloranil as the sensitizer. A transient with an absorption maximum at 500 nm is observed as a short-lived intermediate in both the radical cation mediated 4-methoxystyrene dimerization and the cyclobutane radical cation cleavage experiments. Spectroscopic and kinetic considerations lead to the conclusion that this transient is a substituted hexatriene radical cation produced as an intermediate in the conversion of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation to the dihydronaphthalene. Observed rate constants for the decay of the 4-methoxystyrene radical cation measured at 4-methoxystyrene concentrations of 0.0001 to 0.015 M increase in a linear fashion with concentration, while at concentrations >0.2 M, the observed rate constant is found to be independent of concentration. Analysis of the kinetic data according to a rate law derived from a concerted [2 + 1] mechanism leads to the following rate constants : k(1) = 1.4 X 10(9) M(-1) s(-1) for the addition reaction, k(-1) = 8 x 10(7) s(-1) for the cycloreversion reaction, k(2) = 2.5 X 10(8) s(-1) for the rearrangement of the cyclobutane radical cation, and k(3) = 1.5 X 10(10) M(-1) s(-1) for the reduction of the cyclobutane radical cation by neutral 4-methoxystyrene. Monitoring the kinetics of the reactions of the cyclobutane radical cation gives the same values for the rate constants k(2) and k(-1).