Mixtures of n-decyl-beta -n-glucoside (C(10)G(1)) and n-nonyl-beta-(D)-glucoside (C(9)G(1)) with liquid-liquid closed-loop miscibility gaps display a large deuterium isotope effect when D2O is substituted for H2O, as seen by large shifts in both the critical points and phase boundaries. The observed isotope effect is larger than any previously seen in a surfactant system. It is' suggested that this is:due to both the increased hydrophobicity of the surfactant in D2O and the effect of changes in the hydrogen bond strengths and lengths due to the exchange of O-H for O-D bonds in the glucose headgroups on the spontaneous curvature of the surfactant film.