Refractive indices (n), densities (rho), and surface tensions (sigma) for {formic acid, acetic acid, propionic acid, or butyric acid + water} mixtures at 298.15 K and normal atmospheric pressure have been determined as a function of mole fractions. From the experimental data, excess molar volumes ( V E) and deviations of refractive index from linear behavior (Delta n(phi)) have been calculated. The magnitude of these experimental quantities is discussed in terms of nature and type of intermolecular interactions in binary mixtures. Besides, the interpretation of the ratio of molar volume to molar refraction V/R as a measure of the degree of free volume appears to be a useful tool for qualitative considerations concerning volumetric and refractometric data. To analyze the behavior of surface tensions, the Extended Langmuir (EL) model was used.