The thermal conductivities of 13 rock units of Precambrian, Palaeozoic and Mesozoic age on the island of Bornholm (Denmark) have been measured in situ by the cylindrical thermal conductivity probe technique. Results from the individual Precambrian rocks show narrow distributions with an overall range of 2.4–3.8 W/(m K). As a comparison with theoretical predictions based on mineral compositions indicates accordance within reasonable limits of accuracy, the measured thermal conductivities are considered to be representative for the rocks in question. In contrast to the Precambrian crystalline rocks the younger sedimentary rocks show a wider range of thermal conductivities (1.2–6.2 W/(mK)). In addition to the presentation of results, some special field conditions are considered.