Sodium diclofenac is an analgesic drug commonly used in current therapeutics. Solubility data of this drug in aqueous cosolvent mixtures are not abundant. The main objective of this research was to determine and correlate the equilibrium solubility of sodium diclofenac in some {cosolvent (1) + water (2)} mixtures at 298.2 K. Cosolvents were formamide, N-methylformamide, and N,N-dimethylformamide. Classical shake-flask method was used to determine isothermal solubility. Drug concentration measurements were performed by means of electrical conductivity determinations after adequate aqueous dilutions. Solubility of sodium diclofenac increases non-linearly with the addition of cosolvent to water in almost all mixtures compositions. Higher solubility values were found in {N,N-dimethylformamide (1) + water (2)} mixtures. Jouyban-Acree model correlates solubility values with the mixtures composition for all cosolvent systems. Furthermore, the apparent specific volume of this drug was also calculated from density and mixtures compositions.