A new application of the time-resolved transient grating method for measuring the enthalpy and reaction volume changes in a photochemical reaction is described in detail. From the temporal profile of the signal, these quantities can be determined independently in one particular solvent at a temperature and a pressure. The advantages of this method become clear by a comparison with the previously used photothermal methods, such as the thermal lens and photoacoustic methods. The partial molar volumes of carbon monoxide as well as the reaction volume changes of the photodissociation of diphenylcyclopropenone in several alkanes and aqueous solution are measured, and its solvent dependence is discussed in terms of the isothermal compressibility of the matrix.