Solvent molecules can exert influences on photoinduced isomerization rates, not only in the equilibrium (or static) aspect which involves alteration of the free energy barrier to a reaction coordinate, but also in the non-equilibrium (or dynamic) aspect which originates from the collisional (or viscous) interactions between solute and solvent molecules. The role of dynamic solvent effects is evaluated by studying the solvent viscosity dependence on isomerization rates. The effects of solvent viscosity on the rates of the intramolecular excimer (IE) formation of 3-di(l-pyrenyl)propane (DPP), the ground-state isomerization of 3,3-diethyloxadicarbocyanine iodide (DODCI), and the twisted intramolecular charge-transfer (TICT)-state formation in 4,4`-diamino-diphenyl sulphone (DAPS) in the S_I excited state in a series of alcohol solvents were investigated as the model of one-dimensional barrier crossing by using high pressure techniques. The observed viscosity dependence was analyzed in terms of Kramers theory or Grote/Hynes theory.  Barrier crossing data, i.e., barrier top frequency (ω_b) and barrier energy (E_o), were obtained. Activation volumes due to dynamic solvent effects were also determined.