The permafrost carbon store is increasingly vulnerable with ongoing climate warming. The positive feedback of greenhouse gas release on thawing permafrost is not yet fully understood. One of the main unknowns concerns respiration rates of organic matter contained in permafrost deposits. Respiration rates in thawing permafrost depend on geochemical, microbial and ecological parameters and can be estimated by laboratory incubation experiments. In this study, we compared two 20-m-long sediment cores from Siberian thermokarst lakes. The sediments from one core have been thawed multiple times, whereas the sediments from the second core have been frozen for tens of thousands of years before the lake formed. We incubated 10 g of sediment in glass bottles in an incubation cabinet at 4°C. Anaerobic CH4 and CO2 concentrations were measured every two weeks. We hypothesize respiration rates to be highest in the second core, where the labile fraction of organic matter is still preserved. Furthermore, maximum respiration rates are expected in the first 100 days of the incubations, due to the respiration of the labile fraction of the organic matter. Preliminary results show an increase of CO2 concentration in the first 10 weeks. CH4 concentrations remained low. A later onset of CH4 production has been shown in previous studies, due to the need of establishing methanogenic communities. The CH4 production could become more important in the long-term incubation experiments.