Applications such as CO₂ capture and flue gas dehydration require membranes that are mechanically robust at elevated temperatures and that have high selectivity for CO₂ and/or water over other gases. Polystyrene-b-poly(ethylene oxide) is an amphiphilic block copolymer that satisfies these requirements. This is due to the mechanical strength of glassy polystyrene (PS)  and the hydrophilicity of poly(ethylene oxide) (PEO). The authors have previously shown that PEO indeed is selective for CO₂ over other gases¹ as well as quantifying the transport of water through PS-b-PEO block copolymers^2-3. An important consideration that has not been investigated is the impact of water presence on the properties of PS-b-PEO membranes. We report our findings on the effect of water vapor on the melting temperature of PEO, the glass transition temperature of PS, and the resulting impact on the mechanical properties of the block copolymer. Humidity-controlled differential scanning calorimetry and dynamic mechanical analysis were used to determine these properties. Effective medium and thermodynamic theories are used in an attempt to more fundamentally understand these results.