The influence of wood aqueous extractives on the hydration of plaster is discussed in this paper. Two species were chosen: poplar and forest pine, and the effect of a thermal treatment, which acts as a dimensional stabiliser for wood, was considered. Calorimetric measurements prove that delays of the setting must be expected, especially for the retified poplar. Conductimetric experiments show that wood extractives seem to act as a retardant of the nucleation phase of the hydration reaction. The chemical analysis of the extracts reveals the presence of acetic acid and phenolic molecules, but other experiments prove that their action is not sufficient to explain the retarding effect. Special attention should be given to the influence of polyphenolic substances known as tannins.