Microstructure and interactions in a ternary nonionic microemulsion, composed of pentaethylene glycol dodecyl ether (C-12E5), heavy water (D2O), and decane, were investigated near the phase boundary where the microemulsion is in equilibrium with excess oil using NMR self-diffusion together with static and dynamic light scattering. The system has a constant surfactant-to-oil ratio (51.9/48.1, by weight) and the total volume fraction of surfactant and oil, PHI, is varied in the range 0.01 < PHI < 0.35. From molecular self-diffusion data the microstructure is shown to consist of discrete, oil-swollen micelles in the whole dilution range. The microstructure and phase equilibria are discussed in terms of the flexible surface model which predicts a phase separation involving spherical oil-swollen micelles in equilibrium with excess oil. The self-diffusion and collective diffusion data extrapolated to infinite dilution are consistent with a hydroclynamic radius r(H) = 95 angstrom. The osmotic compressibility, measured by static light scattering on absolute scale, is in good agreement with that of a hard sphere fluid, with a hard sphere radius r(HS) = 86 angstrom and a hard sphere volume fraction given by PHI(HS) = 1.14PHI. The concentration dependence of the self-diffusion and collective diffusion coefficients is similar to those observed in colloidal hard sphere suspensions and shows a good agreement with theoretical predictions for a hard sphere fluid.