Basing on electrical and nonstoichiometry data, defect structure model of Mn1=yS sulphide has been developed. It has been shown, that depending on the temperature and sulfur activity this sulfide may be an electronic and ionic semiconductor. Self-diffusion of cations in metal-deficit Mn1-yS proceeds by simple vacancy, and in metal excess Mn1+yS by collinear interstitialcy diffusion mechanisms.