This work investigated the kinetics of carbon (C) mineralization when different doses of organic materials with varying degrees of stability were added to an arid soil. Respiration assays showed that the incorporation of wastes led to a greater emission of carbon in the form of CO2and greater degree of microbial activity than those occurring in the control soil. Soils treated with fresh waste (municipal solid waste and sewage sludge) gave off more CO2than that treated with compost, with higher values being obtained at high than at low doses. Carbon dioxide emission was reduced with the length of time the organic materials remained in the soil. The data of cumulative CO2were fitted to the equation C=C0(1‐e‐Kt)+C1. The parameters derived from this model were used as indices of organic matter decomposition, because the product of C0and K was more precise than either value separately. In all cases, an initial rapid phase of mineralization was clearly differentiated from a second slower phase.