The stability constants for complexation of the Cu(2+) ion with N-benzoyl-N-methyl-N'-phenylthiourea have been determined at different wavelengths in the range 240 to 265 nm in 0.02 mol dm(-3) of sodium chloride as a background salt in the mixed solvent system of 75% THF + water (v/v) in the pH ranges 5.90 to 7.30 and 9.42 to 10.12 with high ligand-to-metal ratios, by a combination of spectrophotometric and potentiometric methods at 23.5 +/- 0.5 degreesC. Our studies have shown that the ligand forms mononuclear 1:1 and 1:2 complexes with the Cu2+ ion. The protonation constants of the ligand were also determined.