Measurements of water vapor transmission rate (WVTR) are often based on the wet cup method described by ASTM E 96-95. In attempting to compare the performance of thin polymer films with moderate to high water vapor permeability, it was observed experimentally that the ASTM method did not give reliable results for highly permeable films. In particular, the WVTR depended on film thickness and the ratio of film area to water surface area. It was determined that the high water vapor flux through the more permeable films caused a reduction in the driving force for water vapor transmission, that is, the relative humidity at the inner surface of the film. Consequently, the WVTR was underestimated. Comparisons based on a small area ratio and a constant small-flux condition were considered as alternative approaches for evaluating performance using the wet cup method. The constant flux approach produced the best correlation with WVTR that was measured with a commercial instrument.