Meropenem exists in the solid state as trihydrate and methylate. In the present work, the solubility data of meropenem in mixed methanol-water solvent ranging from 273.15 K to 288.15 K were measured and pseudopolymorphic transitions of meropenem were first observed. The transition points were determined at various temperatures. The corresponding water/methanol activities at transition points were calculated using Wilson equation, and correlated by van't Hoff equation. The results show that the ratio of water activity to methanol activity plays a crucial role in determining the thermodynamic stability of trihydrate and methylate of meropenem at a specific temperature. The result can be extended to the application of other pseudopolymorphic systems. (C) 2013 Published by Elsevier B.V.