The accurate evaluation of the activity of water in aqueous phase is of importance in predicting the hydrate formation conditions for polar inhibitor containing systems. In this work, the Patel-Teja equation of state (PT EOS) coupled with the Kurihara mixing rule was used to correlate the gas solubility data and determining the liquid phase composition. Binary interaction parameters between typical natural gas components and hydrate inhibitors have been determined. The PT EOS was then coupled with the hydrate model proposed by Chen and Guo [Chem. Eng. J. 71 (1998) 145] and applied to predict the hydrate formation conditions of various systems containing polar inhibitors. Comparisons with experimental data and the prediction results of typical van der Waals-Platteeuw type hydrate models showed that significant improvement in predicting the hydrate formation of inhibitor containing systems has been achieved.