The effects of "inert" counterions on some photochemical processes involving cationic polymethine dyes have been studied in ion pairs formed in low-polar media. In particular, the counterion effects on the back isomerization kinetics of photoisomers of benzimidacyanine and indotricarbocyanine dyes and on the triplet state decay of the benzimidacyanine dyes have been investigated by flash photolysis. The increase in the rate constant and decrease in the activation energy of back isomerization of benzimidacyanines are observed when dye-counterion ion pairs are formed in low-polar media, whereas the increase in the activation energy is observed for indocyanines under the same conditions. For benzimidacarbocyanine with the I- counterion, ion pair formation gives rise to an increase in the decay rate constant of its triplet state due to interaction of the dye cation with the I- heavy atom; meanwhile, there is no appreciable rise in S-1 --> T intersystem crossing. The enhancement of the S-1 --> S-0 internal conversion in ion pairs has been revealed for all benzimidacyanines with the I- counterion.