A versatile guarded parallel-plate instrument for the measurement of the thermal conductivity in steady state is presented. The instrument stands out due to its applicability for fluids as well as solid bodies, bulk solids, and compressible solids. Besides flexibility regarding sample material and thickness, the instrument design is aimed at the minimization of parasitic heat flows due to convection, radiation, or heat leakages. Remaining effects of radiative heat transfer can be taken into account by appropriate calculation methods. A precise and automated measurement and control system for temperature and heat flow including objective criteria for identifying steady-state conditions allows convenient and accurate measurements. A comparison of results obtained for water, toluene, air, and a PTFE disk at temperatures between 308.15 K and 368.15 K with reference data confirms that uncertainties of less than 3% can be achieved by choosing adequate sample thicknesses and temperature differences between the plates. For compressible materials, the accessibility of the equivalent quantities overall heat transfer coefficient, overall thermal resistance, and apparent thermal conductivity in dependence on the degree of compression is demonstrated. (C) 2012 Elsevier Ltd. All rights reserved.