The solvation number and hydration constant of Th4+ in ethanol-water medium were determined at 25°C using UV-spectral and electrochemical measurements. A solvate formation equilibrium is demonstrated and characterized. Three molecules of ethanol (S) can bond to the metal cation with strengths comparable to that for H2O to form ThS3(H2O)34+ Formation of thorium monochelate with lawsone (2-hydroxy-1,4-naphthoquinone) eliminates bonding with alcohol molecules. The dissociation constant of the chelating agent 5Ka and the formation contant of the monochelated metal ion ,K1* that are essentially independent of the solution composition are evaluated. Hydration titrations involving thoriumlawsone monochelate are performed and the data obtained from the changes of pH with solvent composition are analyzed. The solution independent constant, sKf* for thorium-lawsone complex formation in mixed aqueous ethanol is given by log sKf* = vpKa + logsKh - log [LH] - vpH + 3 log v where vpKa is the dissociation constant of the chelating agent LH in the solvent system of v volume fraction of water and sKh is the solution-independent hydration constant of thorium (IV) in the solvent system. Log-values for the constants sKh, sKf* and sKa* are found to be 7.8±0.02, 11.38+0.04 and -0.753, respectively.