Carbon isotopic fractionations in plant materials and those occurring during decomposition have direct implications in studies of short-and longer-term soil organic matter dynamics. Thus the products of decomposition, the evolved CO 2 and the newly formed soil organic matter, may vary in their 13 C signature from that of the original plant material. To evaluate the importance of such fractionation processes, the variations in 13 C signatures between and within plant parts of a tropical grass (Brachiaria humidicola) and tropical legume (Desmodium ovalifolium) were measured and the changes in 13 C content (signatures) during decomposition were monitored over a period of four months. As expected the grass materials were less depleted in 13 C (À11.4 to À11.9%) than those of the legume (À27.3 to À25.8%). Root materials of the legume were less (1.5%) depleted in 13 C compared with the leaves. Plant lignin-C was strongly depleted in 13 C compared with the bulk material by up to 2.5% in the legume and up to 4.7% in the grass. Plant materials were subsequently incubated in a sand/nutrient-solution/microbial inoculum mixture. The respiration product CO 2 was trapped in NaOH and precipitated as CaCO 3 , suitable for analysis using an automated C/N analyser coupled to an isotope ratio mass spectrometer. Significant depletion in 13 C of the evolved CO 2 was observed during the initial stages of decomposition probably as a result of microbial fractionation as it was not associated with the 13 C signatures of the measured more decomposable fractions (non-acid detergent fibre and cellulose). While the cumulative CO 2 -13 C signatures of legume materials became slightly enriched with ongoing decomposition, the CO 2 -C of the grass materials remained depleted in C. Associated isotopic fractionation correction factors for source identification of CO 2À C varied with time and suggested errors of 2-19% in the estimation of the plant-derived C at 119 days of incubation in a soil of an intermediate (À20.0%) 13 C signature. Analysis of the residual material after 119 days of incubation showed little or no change in the 13 C signature partly due to the incomplete decomposition at the time of harvesting.