Abstract The simulation of cometary surface conditions by laboratory experiments has proven to be an efficient way towards a better understanding of physical phenomena at and below the surface of a comet nucleus (Grün et al., Geophys. Res. Lett. 18, 245–248, 1991). A question which has not yet been answered by the comet simulation (KOSI) experiments performed so far is the influence of organic matter on the physical properties of the sublimation residua. Therefore, a number of experiments performed in a small vacuum chamber cooled by liquid nitrogen are reported on, which were dedicated to study the influence of organics on the thermal properties of a cometary analogue sample. Using aliphatic hydrocarbons of low volatility (paraffins) as model substances for the organic compounds, the formation of a several centimetres thick cohesive residuum in response to heating of the sample was observed. In one of the experiments the evolution of an originally homogeneous multi-component sample (containing water ice, organics, and minerals) to a residuum containing (finally) only minerals and organics was followed. During this evolution the thermal properties changed dramatically. The heat conductivity of the cohesive residuum was found to be at least an order of magnitude larger than the typical value for a loose dust mantle containing no organic material. Thus the evolution of comets with the same thermal history containing a considerable amount of organics might be quite different from that of a comet consisting only of ices and minerals.