Relative reaction rates in selected nuclides have been measured through the central lattice cell of ZEEP with the fuel cluster replaced by a pressure tube containing different mixtures of light and heavy water. The surrounding heavy water moderator was at room temperature. Measurements were made with the pressure tube filled with air and four mixtures ranging from reactor grade heavy water to light water, at temperatures up to 280°C. The radial variations are given for the reaction rates, which were measured, and for the effective neutron temperature T and the epithermal index , deduced from the reaction rates.