Carbon isotopic composition of soils subjected to C 3 -C 4 vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C 4 plant MiscanthusÂgigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The d 13 C and d 15 N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190°C mainly reflected water evaporation because no significant C and N losses were detected and d 13 C and d 15 N values of the residual SOM remained unchanged. The d 13 C values ()16.4&) of SOM fraction decomposed between 190 and 390°C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (d 13 C=)11.8&) compared to the d 13 C values ()16.8&) of SOM fraction decomposed above 390°C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to d 13 C, the d 15 N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for d 13 C.