The transport properties of alpha-Ag2Se at 160 degrees C as a function of nonstoichiometry have been measured in the whole range of homogeneity using a symmetric solid state electrochemical cell with ionic and electronic probes as well as ionic and electronic electrodes. Chemical diffusion coefficients as well as ionic and electronic conductivities were obtained from polarization experiments with high stoichiometric resolution. The composition of the sample was varied in situ by means of the coulometric titration technique. The chemical diffusion coefficient of alpha-Ag2Se shows a distinct maximum in the vicinity of the stoichiometric point with values between 0.04 and 0.12 cm(2) s(-1). The ionic conductivity of alpha-Ag2Se is constant within the whole range of homogeneity with values around 2.2 S cm(-1), whereas the electronic conductivity increases almost linearly from about 600 to 2600 S cm(-1) with increasing silver activity.