The chemical diffusion coefficient and point defect concentration in metal-deficit manganous sulphide (Mn1-yS) has been studied using re-equilibration kinetics and Rosenburg's method. Measurements have been carried out in a microthermo-gravimetric apparatus of a new generation, enabling to make rapid changes of sulphur vapour pressure in the reaction chamber and determine the mass changes of a given sample with the high accuracy, up to 10(-7) g. Using this device deviations from stoichiometry, chemical diffusion in metal-deficit manganous sulphide was investigated, as a function of temperature (973-1273 K) and sulphur activity (10(-2)-10(4) Pa). From obtained data, the self-diffusion coefficient of cations in Mn1-yS has been calculated as a function of temperature and sulphur activity.