Thermal conductivity (λ) of a formation can be determined in situ by pouring or injecting liquid at an arbitrary temperature into a thermally equilibrated borehole and analyzing dissipation of the resulting thermal pulse. We used this technique in a shale by pouring diesel fuel into a borehole and obtained values for λ within a few percent of the mean value obtained from cores. The test showed that the technique is relatively simple to apply and can readily be used to test multiple horizons simulatenously. A priori estimation of formation specific thermal capacity was necessary to obtain good results, apparently because the form of the thermal pulse was not well characterized in our test. This resulted from adding liquid to the borehole in a poorly controlled manner; injecting liquid using a tremie pipe would provide a better characterized pulse and is probably advisable when implementing this technique. ¿ American Geophysical Union 19