The polymorphic transformations of phenylbutazone from metastable forms alpha and beta to stable form delta were studied quantitatively at four temperature and five humidity levels by X-ray powder diffractometry. The transformation of form alpha conformed with the Avrami-Erofe'ev kinetic model and form beta conformed with apparent first-order kinetics. In the two transformation systems, the induction periods depended on the storage conditions and were prolonged with lowering of temperature and humidity. The transformation rate of form alpha was not affected by humidity, whereas that of form beta increased according to a rise in humidity. The temperature dependency of the transformation rate constant was remarkable. The Arrhenius treatment was applicable to the beta----delta transformation at low temperatures. The overall half-life, including induction period, revealed that form alpha was more stable than form beta under any storage condition. A good linear relationship existed between the induction period and the transformation rate constant, irrespective of the storage conditions. The scanning electron photomicrographs of forms alpha and beta demonstrated that acicular crystals of form delta grew as the transformation progressed. This could be confirmed as the change in particle diameter of the samples.