The structure of water entrapped in AOT/PEG 400 reverse micelles was investigated and compared with that of simple AOT reverse micelles by using Fourier transform infrared spectroscopy (FT-IR) to measure the stretching vibrations for O-H, carbonyl (C=O) and SO3- (S=O) bonds. The water in a reverse micelle consists of trapped water, bound water and free water. The fraction of free water in the water pool of reverse micelles containing PEG increases with the molar ratio of AOT to H2O, W-o more steeply than that of simple reverse micelles, and the structure of bound water is disrupted by the addition of PEG. The shift of the peak of the S = O bond and analysis of the C = O bond spectra indicates that the interaction of AOT molecules with water is decreased by hydrated PEG molecules at W-o > 6, which coincides with the results of O-H spectra.