Effect of pressure on the excited-state isomerization rate of 2-(2-propenyl)anthracene in a series of alkane solvents was examined by steady-state and picosecond time-resolved fluorescence spectroscopy. The viscosity dependence of the isomerization rate constant was analyzed by the theories of the dynamic solvent effect on chemical reactions. The viscosity dependence is sufficiently smaller than that predicted by Kramers theory. The experimental result was analyzed by the frequency-dependent friction defined in the Grote-Hynes theory,and the barrier top frequency (omega(b) = 9.6 x 10(12) s(-1)) was determined.