A detailed study of the competition between C-C, C-H, and C-Si bond fragmentation in a series of 4-methoxy-alpha-substituted toluene radical cations (1(.+)), involving both product studies and kinetic analysis, is presented. C-C bond fragmentation occurs with several radical cations in acetonitrile. The rate constants for such processes, determined by laser flash photolysis, varied from 2.8 x 10(4) (1c(.+)) to 1.53 x 10(6) (1f(.+)) s(-1). The activation parameters for C-C bond fragmentation are characterized by low activation enthalpies on the order of 30 kJ mol(-1) and negative activation entropies in the range -34 to -55 J mol(-1) K-1. Deprotonation of the radical cations is always a second-order process induced by nucleophiles [cerium(IV) ammonium nitrate (CAN) or nitrate anion], with second-order rate constants from 7.7 x 10(7) (1h(.+)) to 8.8 x 10(8) (1i(.+)) M-1 s(-1) in neat acetonitrile (CAN assisted) and from 0.4 x 10(8) (1j(.+)) to 7.1 x 10(8) (1i(.+)) M-1 s(-1) in the presence of nitrate anion. The rate constant for nitrate-induced decarboxylation was higher, 13.6 x 10(8) M-1 s(-1) (1d(.+)). In a few cases C-C (1e(.+), 1f(.+)) and C-Si (1g(.+)) fragmentations occurred, also as second-order processes induced by nitrate, with rate constants from 4.4 x 10(8) (1f(.+)) to 8.2 x 10(8) (1g(.+)) M-1 s(-1). Delta H double dagger and Delta S double dagger had opposing influences on C-H and C-C fragmentation, and in the case of 1e(.+) a temperature-dependent product distribution was obtained. The activation parameters for the observed C-H, C-C, and C-Si fragmentations have been compared, and suggest a rationale for the mechanisms and selectivity of such processes in radical cations.