Defect mobility in non-stoichiometric cobaltous sulphide, Co9-xS8, has been studied as a function of temperature (873-1073 K) and sulphur activity (10(-4)-10(-1) Pa) using microthermogravimetric re-equilibration technique. Chemical diffusion coefficient calculated from oxidation and reduction runs were virtually the same, clearly indicating that the rate determining step of the overall re-equilibration process is the solid state diffusion. It has been found that the chemical diffusion coefficient does not depend on sulphur activity and thereby on defect concentration and can be described as a function of temperature by the following empirical equation: (D) over tilde = 3.1.10(-2) exp((-76 kJ/mol)/RT).