Lidocaine.HCl is the most widely used anaesthetic drug in pharmaceuticals intended for local administration. Solubility data of this drug in aqueous cosolvent mixtures is not abundant. The main objective of this research was to determine and correlate the equilibrium solubility of lidocaine.HCl monohydrate in some {cosolvent (1) + water (2)} mixtures at 298.2 K. Cosolvents were polyethylene glycol 200 (PEG 200), ethanol and propylene glycol. Classical shaken-flask method was used to determine isothermal solubility. Concentration measurements were performed by means of electrical conductivity determinations after adequate aqueous dilutions. Solubility of lidocaine.HCl.H2O decreases non-linearly with the addition of cosolvent to water in almost all mixtures compositions. Jouyban–Acree model correlates solubility values with the mixtures composition for all cosolvent systems. Apparent specific volume of this drug was also calculated from density and mixtures compositions.