The water-vapor permeability of poly(octadecyl acrylate) (PA-18) was measured as a function of temperature in the region traversing its melting point (50 degreesC). The molten-state permeability of PA-18 is comparable to that of shorter side-chain methacrylate polymers. Water permeability in the semicrystalline state of PA-18 is similar to that of polyethylene at comparable crystallinity levels. The permeation switch, or change in permeability with the traversing of the melting point, for water is discussed in the context of previous results for other penetrants in this and other side-chain crystalline polymers.