The preparation and characterization of humidity-indicating silica gel containing copper sulfate were aimed at in the present study. The equilibrium and kinetics of Cu(II) sorption were investigated by contacting 200-1600 mg dm(-3) Cu ion-containing solutions with silica hydrogel slabs having 10% silica. The diffusion coefficient of Cu(II) in silica hydrogel was determined to be on the order of 10(-10) m(2) s(-1). X-ray diffraction indicated that the gels contained mainly CuSO4 center dot 3H(2)O, CuSO4 center dot H2O, and small amounts of CuSO4 and CuSiO3 center dot 2H(2)O. The silica gel with Cu(II) can be used as a humidity indicator since its color changes from light blue to dark blue upon moisture adsorption.