Four bis-oxonols have been synthesized which possess different alkyl substituents appended to the thiobarbiturate subunit. The nature of the alkyl substituent affects the photophysical properties of the dye in solution since it modulates the rate of rotation of the thiobarbiturate subunit around one of the double bonds in the connecting trimethine bridge. Rates of light-induced (trans to cis) and thermal (cis to trans) isomerization processes have been measured for one of the dyes in protic (i.e. alkanols) and aprotic solvents at various temperatures. These rates, together with activation energies derived from Arrhenius plots, are discussed in terms of the hydrogen bonding and stereochemical properties of the solvent. The thermal step is very sensitive to the stereochemistry of the solvent while the light-induced process is controlled mostly by the size of the rotor and the solvent viscosity. The overall effects may be explained, at least in a qualitative sense, in terms of the medium-enhanced barrier model.