In the hydration of α-pinene, catalysed by an H2SO4 aqueous solution, a solvent such as isobutyl alcohol can be used as a solubility promoter. The consumption of α-pinene shows an apparent deviation from first-order kinetics. However, if the temperature is sufficiently high to avoid terpine formation first-order kinetics are observed. To explain this behaviour, a kinetic model was derived which takes into account the equilibrium between the phases and assumes that the chemical reaction takes place only in the aqueous phase and that the volume of the phases may change during the reaction. The parameters of the model were estimated by non-linear regression analysis. The good fit of the model to the experimental points allows the deviation from first-order kinetics, observed at low temperatures, to be explained by the change in volume of the phases during the reaction and the different solubilities of the species in the two phases.