The silver chalcogenides alpha’-Ag2+deltaTe and beta-Ag2+deltaSe represent two contrary examples regarding the composition dependence of the chemical diffusion coefficient. Whereas in the vicinity of the stoichiometric composition the coulometric titration curve of beta-Ag2Se is determined by the chemical potential of the silver ions (due to its cationic Frenkel disorder), the coulometric titration curve of alpha-Ag2Te is determined by the chemical potentials of the electrons (due to its structural disorder). The experimentally determined maximum of the chemical diffusion coefficient, in the case of alpha-Ag2+deltaTe, near the stoichiometric point as well as the monotonic decrease of the chemical diffusion coefficient with decreasing silver activity in the case of beta-Ag2+delta Se, can be explained in detail.