A thermodynamic model based on sub-regular solution was utilised to facilitate prediction of high temperature activities of constituent elements in austenitic stainless steels and other iso-structural alloys. Nearly 95 interaction parameters needed for the computations were also compiled from the literature. The model was utilised for computing activities of a few major metallic components such as Fe, Cr, Ni, Mo and Mn in most of the austenitic stainless steel series. A comparison was made between the activities predicted by the model and those reported from the available literature which were based on Knudsen cell mass spectrometry and metastable EMF methods, essentially on some Fe–Cr–Ni alloys and stainless steel type AISI 304 and 316. The possible reasons for the mismatch in the values of activities were discussed. The application of the model for simulation studies involving variation of composition with respect to one solute element was also presented.