Demixing in an atmospheric‐pressure free‐burning welding arc was investigated using a two‐dimensional numerical model. Simulations of arcs in argon–nitrogen and argon–helium mixtures demonstrated that demixing can have a major influence on the composition of the arc in both the central and edge regions. Convection suppressed the change in composition in the central 0.5 mm of the argon–nitrogen arc only. Furthermore, demixing was found to have only a relatively minor effect on temperature and axial velocity, but it significantly altered thermal transport in the argon–nitrogen mixture. It is concluded that demixing can be important in both chemical and thermal applications of mixed‐gas arcs. ©1996 American Institute of Physics.