The trans → cis photoisomerization of a series of substituted stilbazolium salts (A t + X − , A t + : trans-1-alkyl-4-[4-R-styryl]-pyridinium and -quinolinium, R = CN. H, CH 3 and OCH 3 , X − = I and ClO 4 − ) was studied by laser flash photolysis and steady state irradiation measurements. The quantum yields of cis ⇄ trans photoisomerization (ϕ c → t and ϕ t → c ) and of fluorescence of the trans isomers (ϕ f ) were determined in several solvents at room temperature and at low temperatures in mixtures of either 2-methyltetrahydrofuran-dichloromethane or ethanolmethanol (E-M). In polar solvents at room temperature ϕ t → c is substantial ( ≧ 0.3) and ϕ f is small (10 −3 -10 −2 ). Competition of fluorescence and an activated step in the trans → cis pathway is indicated by the effects of temperature on ϕ f and ϕ t → c (activation energy: 2 - 3 kcal/mol). A transient, observed only at low temperatures (lifetime τ T > 0.5 ms in E-M below -170°C), is assigned to the lowest triplet state with trans configuration. On the basis of the effects of temperature on ϕ f , ϕ t → c , and the triplet yield and those of quenchers on ϕ f and ϕ t → c , involvement of the triplet state in the twisting process at room temperature is excluded. Therefore, a singlet mechanism is suggested for the trans → cis photoisomerization of the stilbazolium salts examined. Significant reduction of ϕ t → c for iodides in solvents of moderate polarity, where ion pairs are present, is accounted for by photoinduced electron transfer in competition to trans → cis photoisomerization.