The absorption spectra of 0.5 aquamolality solutions of copper sulfate (CuSO 4 ) in H 2 16 O, D 2 16 O and H 2 18 O were determined at room temperature in the wavelength range of 400 – 1100nm. Both deuterium (D) and 18 O isotope substitutions resulted in a decrease of 6% and 1%, respectively, in the integrated absorption coefficient. No shift in the maximum of the absorption band was observed; the absorption band became narrower. The tint of the D 2 16 O solution is slightly greener than those of the H 2 18 O and H 2 16 O solutions. It is suggested that the observed isotope effects on the absorption spectra may mainly be ascribed to the change in the closest approach of the first coordination sphere water molecules to the Cu 2+ ion brought about by isotope substitution.