In the paper, steels are studied that are mostly used in technical practice as base materials, filler metals, and consumable electrodes in the design of dissimilar weld joints. These steels differ mainly in the content of Cr, Mo, V, W, and N. It is explained and emphasized in the paper that the effect cannot be underrated of thermodynamics, diffusion and phase transformations on the stability of dissimilar weld joints that are exposed to temperatures of over ca. 500degC during operation. For all the materials under study the carbon activity at 600degC is given as calculated by the CALPHAD method, and a correlation is shown to exist between the carbon activity and the total chromium content. This correlation can be used as a most general alternative in the selection of filler metals or consumable electrodes in the design of dissimilar weld joints. The primary goal of the paper is, however, to present a more sophisticated alternative based on a mutual comparison of the temperature dependence of the carbon activities of the base alloys, filler metals and consumable electrodes under consideration. This alternative is suitable for routine engineering purposes. Attention is also drawn to the currently most advanced alternative, which enables predicting the redistribution of elements and phases after prolonged exposure.