This study is concerned with the determination of the diffusion coefficient of water into polymer films using a thermogravimetric analyzer (TGA). First, the sample film in the sample chamber was dried completely by N2 gas supplied by a cylinder. Next, the mixture of air and N2 gas having a given humidity was introduced into the sample chamber of the TGA. The sample weight was recorded as a function of time. The diffusion coefficient was calculated from the initial slope of weight vs. the square root of the time curve. The results were consistent with those obtained using Prager and Long's method. © 1993 John Wiley & Sons, Inc.