A simple method to measure a rate of the translational temperature increase after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the nonradiative transition. Using this method, the thermalization rate after the photoexcitation of trans-azobenzene to the S-2(pi pi*) State is measured. The acoustic peak shift indicates that the temperature of the solvents rises within less than similar to 3 ps after the nonradiative transition of the S-2 --> S-1 process. The thermalization after the S-1 --> S-0 transition completes faster than the decay of the S-1 state (16 ps in acetonitrile and 18 ps in ethanol). The fast energy transfer from the solute to the solvent is discussed.