Among a variety of origins of the transient grating (TG) signal, the thermal effect and molecular volume effect are prominent origins for studying photochemical reactions in solution phase. Density change due to the thermal expansion and/or the molecular volume change can be sensitively and quantitatively detected by the TG technique with a high temporal resolution continuously from the light irradiation. The time-resolved measurements of the volume change and the enthalpy change upon photochemical reactions can be performed by analyzing the TG signal. The principle, merits and disadvantages of this method are reviewed.