Information about the reaction in which β-molybdosilicic acid is transformed into the α-acid is required in the context of both general chemical studies of the molybdosilicic acids and analytical studies. Previously, only Massart seems to have studied the reaction kinetics in any detail. Although our study confirmed Massart's observation that the rate of transformation of β-acid is first order with respect to β-acid concentration, it supported an alternative and simpler explanation for the changes in the apparent rate constant that accompany changes in the hydrogen ion concentration in the reaction mixture. The mechanism consists of a unimolecular transformation reaction with an acid-base pre-equilibrium involving the transformable β-acid species: when the pH is lowered, the shift in the acid-base equilibrium reduces the proportion of the total β-acid concentration present as the transformable species and thereby decreases the apparent rate constant. The apparent rate constant was found to be zero order with respect to molybdate concentration (0.005–0.050 M molybdate-molybdenum). The activation energy of the elementary reaction was found to be 15.7 ± 1.2 kcal mol^–1. The study has shown that the α-acid decomposes at pH values less than approximately 0.75 and greater than 3.5 (in 1.0 M sodium chloride solution). It is suggested that the β-acid decomposes more rapidly than the α-acid at any given pH less than 0.75. Methods of non-linear parameter estimation have been used extensively in fitting theoretical models to observed data.