Photolysis of N-nitrosamines in acidic acetonitrile produces aminium radical cations via protonation of the initially-formed aminyl radicals. The kinetics of these species can be monitored by transient UV spectroscopy via their absorption band which is found at ca. 300 nm in the case of the piperidinium radical, for example. By measuring the aminium radicals’ lifetimes as a function of the concentration of added olefin, absolute values for the bimolecular rate constants for the addition reactions were obtained. In the case of the piperidinium radical, these rate constants varied from <1 x 10(6) M(-1) s(-1) for acrylonitrile to 1.1 +/- 0.1 x 10(9) M(-1) s(-1) for 1,1-diphenylethylene and generally increased with decreasing ionization potential of the olefin, thus confirming the electrophilic nature of the piperidinium radical. The rate constants for analogous reactions of diethylaminium radicals were 1.5-25 times smaller indicating the importance of steric factors in aminium radical additions to olefins. The rate constant for the intramolecular 1,5-addition of the secondary aminium radical cation to an unactivated double bond is estimated to be ca. 1 x 10(6) s(-1), but the intramolecular addition rate constant increases to >1 x 10(8) s(-1) upon the phenyl substitution at the olefinic terminus.