Tracer diffusion coefficients of copper(II) in HCl and HClO₄(0·25–10M) were measured by the radiometric porous-frit method. Complexes of Cu^II with chloride ions appear to account for the differences observed in the two acid systems. A comparison of the relative diffusion coefficient–viscosity products of Cu^II, Zn^II, and Sr^II in HCl shows that Cu^II is complexed to a lesser extent than Zn^II while Sr^II does not appear to be complexed. Our measured diffusion coefficient for copper in 1M-HCl agrees reasonably well with the values reported in several chronopotentiometric and polarographic papers in the literature. In perchloric acid solutions (0·25–8M), the relative diffusion coefficient–viscosity products for Cu^II and Zn^II are essentially the same.