In previous evaluations, of GTA welding stainless steel plates the surfactant content in the workplace has been assumed to be in a state of equilibrium. However surfactant concentration usually had to be adjusted in order to achieve agreement between the weld pool shapes in experiments and those in simulations. In this study a physiochemical approach to the redistribution of surfactant at the surface and in the bulk is presented for the first time. Sulphur was considered as a represenative surfactant. The model allows surfactant molecular transport via convection, diffusion, and sorption. It is shown that sulphur atoms accumulate at the surface at stagnation points, affecting the coefficient of surface tension and thus the final weld pool shape. Thus, the sulphur content in the simulation approaches the nominal sulphur content in the steel plate. The theory is strenghtened by both electron probe microanalysis and Auger electron spectroscopy. At the time the work was carried out, Mr Winkler, Dr Inoue and Mr Fuji were at the Welding and Joining Reserach Center, Steel Reserach Laboratories, Technical Development Bureau, Nippon Steel Corp., 20-1 Shintomi, Futtsu, Chiba 293, Japan, Dr Amberg was in the Department of Mechanics, Royal Institute of Technology, Osquars Backe 18, 100-44 Stockholm, Sweden, and Dr Koseki was at the Oita R&D Laboratories, Nippon Steel Corp., 1 Nishinosu, Oita 870, Japan. Mr Winkler is now in the Department of Mechanics, Royal Institute of Technology, Manuscript received 4 March 1999; accepted 8 April 1999.