A systematically derived model is presented for the turbulent combustion of CO/H-2-air mixtures in diffusion flames with high mixing rates. The mixing of fuel and air is expressed by a mixing variable. The conversion of CO to CO2, of H-2 to H2O and of the non-equilibrium intermediate species concentrations are determined by two reaction progress variables. The thermal NO production is calculated in a postprocessing step. In the paper the turbulent source terms in the transport equations for the scalar variables are discussed. A case of a turbulent CO/H-2 diffusion flame with chemical super-equilibrium of intermediate species is presented. The influence of the intermediate species HO2 an H2O2 on the simulated characteristics of the flame is investigated.