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Soil microbes favour easily decomposable organic compounds, and, consequently, soil organic matter tends to enrich with recalcitrant carbon compounds. In theory, this leads to an increasing temperature sensitivity of decomposition towards older fractions of soil organic matter. We tested this theory in a Sphagnum peat profile. This profile was particularly suitable for this purpose because the age of organic matter increased with depth. The age correlated closely with the degree of decomposition in the oxic layer and soil minerals did not interfere with the relationship between the age and the temperature sensitivity of decomposition. We sampled and analysed the peat profile by layer. We took laboratory measurements of chemical characteristics, decomposability and temperature sensitivity of decomposition. We used the bomb-¹⁴C isotopic tracer to differentiate the sources of respired CO₂ into age-classes and to …