Two different approaches for modeling reflectance spectra of intimate mixtures, Hapke's model and the isograin model, are used to estimate grain sizes and mixing ratios of powder mixtures of three geologic minerals: olivine, orthopyroxene, and plagioclase. In Hapke's model, scattering and extinction efficiencies are mixed separately, and both models employ semiempirical refractive index spectra for component minerals. Mixing ratios of mixtures of grain size 45--75 μm are well estimated by both models assuming a common grain size of 60 μm and optimizing the constants for the single-particle scattering. For each model, effective grain size ratios for mineral constituents in mixtures of grain size <25 μm, are derived successfully that allows mineral abundances to be accurately predicted within ~4 wt %. On the other hand, neither model can accurately predict mineral reflectance spectra for its smaller grain sizes (<25 and 25--45 μm) using an absorption coefficient spectrum derived from a larger grain size (45--75 μm). The errors in both models are significantly reduced if surface roughness effects of the smaller grain-size fractions are modeled. ¿ American Geophysical Union 1994