A dual-wavelength on/off-excitation thermal lens technique was used to identify and quantify a laser-induced chemical reaction in ionic aqueous solutions of Fe(II)-TPTZ. On/off modeling was used to fit the TL experimental data, which provided the primary effect generated during laser-excitation. The addition of HCl in the solutions reduced the activation barrier; this behavior followed the Arrhenius correlation. The nature of the photo-oxidation of Fe(II)-TPTZ complex is discussed. The results suggest that this technique may contribute to the understanding of the dynamics of complex reactions, which may lead to a more precise determination of the physicochemical properties involved in a photochemical reaction.