The thermodynamic melting properties (temperature, enthalpy, and entropy) of rosmarinic acid were experimentally determined via differential scanning calorimetry. The solubility of rosmarinic acid in three solvents (water, methyl acetate, and ethyl acetate), and in their binary mixtures, were determined. The data obtained experimentally were correlated by way of a modified Apelblat model. Through solubility measurements, the activity coefficient was calculated for the process of dissolving the rosmarinic acid in pure solvents and in their mixtures.