Relative reaction rates have been measured in the fuel and moderator of a CANDU-type lattice cell and analyzed to obtain the spectrum parameters, i.e., effective neutron temperatureTand epithermal index. Measurements were made with the coolant channels containing the normal heavy water and also light water and mixtures of light and heavy water at temperatures up to 280°C. An empirical formula has been derived for the effective neutron temperature in the fuel of operating power reactors with similar fuel geometries.