Novel methods are described for the measurement of changes in the water content, conductivity, and electrochemical stability of PEG-based polymer electrolyte films containing lithium perchlorate during exposure to various concentrations of water vapour. Results indicate three regimes of water interaction: an aqueous solution at high humidity, a region of continuous solid solution of water based on a salt/water interaction between H2O:Li+ = 1:1 and 4:1, and a region of high water affinity below H2O:Li+ = 1:1. The conductivity changes exponentially with water content according to a combination of a plasticization and ca-solvent effects, so that conductivity is a sensitive probe of water content. The average diffusion coefficient of water, depending on the water content, is estimated to be about 10(-7) cm(2)s(-1). Preliminary results are given for the effects of water on the interface between the polymer and lithium metal.