Abstract The diffusion coefficients of argon in water-rich regions of aqueous solutions of ethanol and of 1,4-dioxane and those of methane in aqueous solutions of ethanol have been measured by using a diaphragm cell apparatus, designed after Hildebrand et al., for dissolved gases in liquid. The diffusivity of gases was found to show much anomaly which might be ascribed to the structural change of water due to the addition of another solvent. The viscosity reduced diffusion coefficient Dη (D: diffusion coefficient, η: viscosity coefficient) has a maximum value in dilute aqueous solution of both ethanol and 1,4-dioxane.