Sodium naproxen is a non-steroidal, anti-inflammatory drug which has been shown to exhibit four pseudopolymorphic forms (anhydrous, monohydrated, dihydrated, and tetrahydrated). The purpose of the present study is to examine the transition of pseudopolymorphic forms of sodium naproxen under mixed solvent (methanol/water and ethanol/water) systems with varying temperature. Using batch cooling crystallization, transitions can be seen clearly through a van't Hoff plot of solubility data. Currently, water activity is thought to play an important role in determining when transitions occur. This study explores the role of water activity in pseudopolymorphic transitions for these mixed solvent systems; specifically comparing the water activity at the various transitions of the pseudopolymorphs and how this change in the different alcohol/water systems.