The effect of moisture uptake has been investigated in 32 mol% ethylene-vinyl alcohol copolymer (EVOH), amorphous polyamide (PA), and nylon-containing ionomer and in a number of binary blends of these polymers by sorption, TGA, DMA, microhardness and tensile testing. Blends of these materials have been claimed by manufacturers to improve the thermoformability characteristics of EVOH while retaining/improving high gas barrier under high relative humidity conditions. From the results water sorption was found to be in the order EVOH > PA much greater than ionomer. The moisture was found to bind more strongly (higher temperature desorption by TGA) with EVOH than with PA or ionomer, but the fraction of non-hydrogen bonded to the polymer (mostly clustered) moisture appeared to be lower for PA than for EVOH. The T-g of water equilibrated PA was above room temperature, whereas it was well below room temperature in water equilibrated EVOH. Accordingly, EVOH showed increased plasticity and toughness at high relative humidity conditions. An intrinsic high stiffness and brittleness was measured for the PA irrespective of relative humidity. The ionomer showed low moisture dependence in the mechanical properties and much higher flexibility than PA. In EVOH/PA and EVOH/ionomer blends, the EVOH fraction was found to be fully plasticized. The phase compatibility suggested from earlier work for some EVOH/ionomer blends appeared to be eliminated by the effect of sorbed moisture on the potential interfacial adhesion via hydrogen bonding. In the mechanical properties of the blends exposed to moisture, EVOH/PA blends displayed increased stiffness but fragility compared to EVOH, whereas high flexibility and toughness was still observed in EVOH/ionomer blends at high relative humidity conditions.