Evolution of the total carbon (C) content and the 13C enrichment (δ13C signature) of soil organic matter (SOM) with increasing depth in a soil profile under permanent grassland (C3 vegetation) were investigated. The relationship between the total C content and the δ13C signature at different depths in the upper 30 cm of the soil profile could be well fitted by the Rayleigh equation (y = −29.8 − 2.3x, R2 = 0.95, p < 0.001), describing the enrichment in 13C as resulting from isotopic fractionation associated with C mineralization (isotope enrichment factor ε = −2.3‰). Potential C dynamics of SOM in four depth intervals of the profile (0–10, 10–20, 20–30 and 30–40 cm depth) were investigated through an incubation study. The C decomposition rate constants decreased with increasing sampling depth from 0.0479 yr−1 (0–10 cm sampling depth) to 0.0256 yr−1 (30–40 cm sampling depth) and were highly correlated (y = 0.02 + 0.13x, R2 = 0.93, p < 0.05) with the corresponding Δδ13C values (average change of the δ13C signature per depth increment). These results suggest that changes of the δ13C signature of SOM in undisturbed soil profiles under continuous C3 vegetation may serve as an indicator of the variation of SOM quality with increasing depth. Copyright © 2002 John Wiley & Sons, Ltd.