An account is given of measurements on the thermal conductivity of superconducting tin below 1 °K, made by observing the conduction of heat along a tin rod joining two pills of paramagnetic salt. Measurements were made on specimens of widely differing purities and a comparison of the results for the several specimens enabled an estimate to be made of the separate contributions, from the electrons and from the lattice, to the conductivity of each specimen. The electronic contribution to the thermal conductivity is in agreement with calculations due to Heisenberg and Koppe. The significance of this is discussed and it is tentatively suggested that the thermodynamic properties of the superconducting state may be determined by an energy gap of order KT_c in the spectrum of energy levels available to the electrons.