The reliability of the transient grating technique to determine the energetics of processes following second order kinetics, such as free ion recombination, is investigated theoretically. As the second order process takes place, the grating of the reactant population becomes anharmonic, complicating the determination of the population dynamics from the time profile of the diffracted intensity. However, the thermal phase grating is anharmonic from the beginning of the reaction, but is harmonic when the process is completed. As a consequence, the energetics of the reaction can be deduced accurately from the maximum amplitude of the diffracted intensity without having to take the anharmonicity of the grating into account, as long as thermal diffusion is slow. To achieve this, the crossing angle of the pump pulses on the sample has to be smaller than 1°. Otherwise, an arduous fitting procedure is required to extract the energetic parameters from the experimental data.