The isotope effect for diffusion of Ag¹⁰⁵ and Ag¹¹¹ has been measured at six temperatures in AgBr, two in AgCl, one in AgBr+0.08% CdBr₂, and one in silver metal. The results for silver metal and cadmium-doped AgBr are consistent with the vacancy mechanism and a 'kinetic energy factor' Delta K equal to 0.86+or-0.05 and 0.66+or-0.02, respectively. A method of analysing the measurements in the pure silver halides is presented. The problem is more complex than for other systems studied to date because of the simultaneous operation of a vacancy and two types of interstitialcy migration processes in AgBr-type crystals. The equation for the mass effect of the total interstitialcy components is derived.