Rotation times of perylene and 8-methoxypyrene-1,3,6-trisulfonate (MPTS) in 1-alkyl-3-methylimidazolium tetrafluoroborate and 1-alkyl-3-methylimidazolium hexafluorophosphate under high pressure have been examined by means of time-resolved fluorescence anisotropy measurements. The isothermal compression of sample solutions by the application of high pressure can change the solvent viscosity alone over a wide range, allowing for an observation of the pure viscosity dependence of rotation times at a constant temperature (45 °C). Rotation times of both perylene and MPTS show a considerably nonlinear dependence on the solvent viscosity and a substantial deviation from the predictions of hydrodynamic and quasihydrodynamic theories, particularly at high pressures.