Polymorphic transformation of DL-Methionine (DL-Met) crystals was carried out in solutions under isothermal conditions above the transformation temperature and the overall transformation rate was discussed. From the changes in the fraction of stable phase in the crystals and the solution concentration, it was concluded that DL-Met crystals exhibited simultaneous solid-state and solution-mediated transformations (SMT) in the solution. Furthermore, the solid-state transformation (SST) from gamma-form to alpha-form occurred even in the solution, however, it was dependent on the properties of the crystals rather than the experimental conditions. The gamma-form crystals newly deposited from the solution due to the SMT and they did not cause SST in the solution even though they were maintained at elevated temperature. Based on these experimental results, a new model was proposed to analyze the overall transformation and this model qualitatively satisfies the experimental results of transformation in the solution. (C) 2002 Published by Elsevier Science B.V.