The accurate measurement of water vapor sorption and permeation in polymers is complicated because water has a tendency to adsorb on high energy surfaces, a relatively high heat of vaporization, and a high solubility in most polymers. These issues and the difficulties they cause in the design of sorption and permeation equipment are reviewed. Some new approaches to circumvent these problems are described. Data for bisphenol A polysulfone films are used to illustrate these approaches.