An apparatus (Transient Divided Bar - TDB) was implemented to determine the thermal conductivity of rocks. TDB consists of a stack of elements, which comprise a rock specimen placed between two copper blocks of cylindrical shape with known thermal capacity, initially kept at room temperature. The lower block is then cooled, and temperature changes of the upper and lower blocks are continuously recorded. The rate at which heat leaves the upper block depends on the rock thermal conductivity. First, TDB was calibrated with silica glass and quartz; then thermal conductivity of several rock types was measured both with this apparatus and a commercial device. Substantial agreement was obtained between the two instruments. An empirical formula was also found for the calculation of the thermal conductivity by using only records of the upper block temperature as a function of time and the room temperature. Finally, the results of thermal conductivity and its relation with porosity and density are presented for a wide set of fine-grained, macroscopically isotropic and homogeneous rocks. (C) 2014 Elsevier Ltd. All rights reserved.