Absolute rate constants for the reactions of styrene, 4-methylstyrene, 4-methoxystyrene, and beta-methyl-4-methoxystyrene radical cations with a series of alkenes, dienes, and enol ethers have been measured by laser flash photolysis. The measured rate constants correspond to either addition or electron transfer reactions, with the latter predominating when the oxidation potential of the alkene is lower than that of the styrene. The measured rate constants for the diene additions provide some of the first absolute kinetic data for the initial step in the synthetically-useful radical cation mediated Diels-Alder reaction. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. For example, the reactivity of the radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH3O-beta-CH3. The effects of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl less than or equal to 2,2-dialkyl < tetraalkyl. The addition of the 4-methoxystyrene radical cation to a series of ring-substituted styrenes gives a reasonable Hammett correlation with a rho value of -5. Thus, the addition of radical cations to a variety of alkenes and dienes follows similar trends to those observed for the addition of other electrophiles, such as diarylcarbenium ions. The results are consistent with previous suggestions of a concerted pathway for radical cation mediated cycloaddition reactions, although direct spectroscopic evidence for the initial product radical cation is obtained only for the additions to substituted styrenes.