Fluorescence quantum yields and emission lifetimes have been measured at room temperature for a series of thiacarbocyanine dyes in various alcohol solvents, All radiationless deactivation events are accounted for by a process, e.g., torsional relaxation, whose rate is viscosity-dependent. The relaxation rate constant (k(eta)) and the exponent of viscosity dependence (alpha) in the Wirtz-Gierer analysis decrease with increasing polymethine chain length (n) in the dye series. Two dyes were further examined by time-resolved picosecond laser flash spectroscopy. Recovery of ground state in both cases is biphasic. In both cases, the short recovery time corresponds to the fluorescence lifetime. We infer that torsional relaxation of the vertical S-1 state is rate determining and does not cross between the S-1 and S-o potential surfaces in the relaxed S-1 conformation. The slower recovery process is identified with a pathway that leads to photoisomerization of the cyanine chromophores and implies involvement of a species intermediate between torsionally relaxed Si and the groundstate photoisomer. This intermediate is suggested to be an isomerized triplet state.