Systematic variations in the carbon isotopes of soil organic matter (SOM) have been observed in a variety of ecosystems. Such variations may provide important insights into physical and biological processes that mediate carbon storage, nutrient availability, and trace gas emissions. To study carbon cycles using carbon isotopes, it is important to link carbon isotope systematics with carbon dynamics models, so that carbon isotope variations can be interpreted in the context of rates and fluxes of carbon transport in ecosystems. This paper presents a theoretical analysis, in which carbon isotopic evolution of decaying organic matter is modeled for a system in which the rate of litter or soil organic matter decomposition is controlled by substrate quality and microbial growth rate. The model examines two mechanisms controlling the isotopic evolution of carbon in a system with a one-time input: kinetic isotopic fractionation by soil microbes and differential decomposition. The model produces several important characteristics of a SOM decomposing system: (1) for a system with a single population of organic matter, the carbon-13/carbon-12 ratio in SOM increases with increasing carbon loss as a result of kinetic isotope fractionation of respiration; (2) for a given isotopic fractionation factor, the isotopic enrichment relative to the original organic material increases with the decomposition rate; and (3) for a system containing multiple organic matter components having different qualities and isotopic compositions (e.g., cellulose and lignin), the temporal evolution of the SOM isotope ratio reflects the competing dominance of isotopic fractionation and preferential reservation of one or more organic components.