A thermodynamic framework is presented which treats the mixing of refractory metal carbides and nitrides, e.g. Ti, Nb and V which appear in microalloyed steels. The intermixing of metals solutes, carbon and nitrogen on a single precipitate crystal lattice is considered, using a quasi-regular solution model. Two extreme cases are considered; complete intermixing of all binary compounds to provide an upper limit for precipitate stability, and mutual immiscibility of binary compounds allowing only the simultaneous coprecipitation of carbides and nitrides. These extreme cases provide an upper and lower bound for carbonitride solubility. Specific examples are used to illustrate precipitation from austenite of a ternary carbonitride, NbC y N 1− y , the quaternary mixed carbonitrides Ti x Nb 1− x C y N 1− y , Ti x V 1− x C y N 1− y and V x Nb 1− x C y N 1− y and the quinery nitride, Ti x Ta y Nb z V 1− x− y− z N. The determination of solution parameters allow the computation of the multi-component phase diagram Fe - Σ i Mi Σ j X j . Finally, the time-temperature constraints or processing are briefly addressed by allowing limited precipitate intermixing, whilst maintaining equilibrium between solutes in austenite.