An electrochemically probed relaxation method has been used to determine the chemical diffusion coefficient, D̃, of α-Ag 2+δS. The experiments were performed at 168°C and span the homogeneity range of this phase, which at this temperature is of the order of 10 -5 in δ. D̃ is found to have values between 0.02 and 0.15 cm 2 s -1 and, as a function of nonstoichiometry, to reflect correctly the composition dependence of the thermodynamic factor, d ln a Ag/d ln c Ag. Using previously determined data for the thermodynamic factor, the component diffusion coefficient of Ag + at 168°C is found to be D Ag + =(5.4 ± 0.4) × 10 -8 cm 2 s -1.