Vibrational relaxation and dissociation of CH3I, 2-20% in krypton, have been investigated behind incident shock waves in a diaphragmless shock tube at 20, 66, 148, and 280 Torr and 630-2200 K by laser schlieren densitometry. The effective collision energy obtained from the vibrational relaxation experiments has a small, positive temperature dependence (down) = 63 x (T/298)(0.56) cm(-1). First-order rate coefficients for dissociation of CH3I show a strong pressure dependence and are close to the low-pressure limit. Restricted-rotor Gorin model RRKM calculations fit the experimental results very well with (down) = 378 x (T/298)(0.457) cm(-1). The secondary chemistry of this reaction system is dominated by reactions of methyl radicals and the reaction of the H atom with CH3I. The results of the decomposition experiments are very well simulated with a model that incorporates methyl recombination and reactions of methylene. Second-order rate coefficients for ethane dissociation to two methyl radicals were derived from the experiments and yield k = (4.50 +/-0.50) x 10(17) exp(-32709/T) cm(3) mol(-1) s(-1), in good agreement with previous measurements. Rate coefficients for H + CH3I were also obtained and give k = (7.50 +/- 1.0) x 10(13) exp(-601/T) cm(3) mol(-1) s(-1), in reasonable agreement with a previous experimental value.