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 temperature T and 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.