Isomerization of 2-vinylanthracene in the first excited singlet state has been studied over wide solvent-viscosity ranges. Steady-state and time-resolved fluorescence spectroscopy is used to determine the isomerization rate in compressed liquid solutions and in supercritical fluids. The "Kramers turnover," which indicates the transition of kinetics from "energy diffusion control" regime to "spatial diffusion control" regime, is clearly observed. The turnover behavior can be expressed by a single curve which bridges the two extremes by an additive form. The activation energy shift between the two extremes is 1.6 kJ/mol. Activation volumes at large viscosity in compressed liquid solutions are also discussed.