The diffusion of copper in "low digenite", Cu1.8S, has been measured by an electrochemical potentiometric method within the temperature range 15-50 degrees C. At 25 degrees C the chemical diffusion coefficient ($) over tilde D is 1.8(1) x 10(-8) cm(2)/s in acidified (pH similar to 2) copper sulfate solution and 8.2(2) X 10(-9) cm(2)/s in copper nitrate/potassium nitrate solution. The intrinsic diffusion coefficient follows D-Cu=0.011 exp(-5500/T) cm(2)/s, respectively 0.11 exp(-6400/T). The 25 degrees C values are 1.1 X 10(-10) cm(2)/s and 4.8 x 10(-11) cm(2)/s. The method reproduces a previous value, D = 0.036 exp(-6100/T), obtained by other means.