Information about effective neutron temperatures has been inferred from measurements of the ratio of the thermal-fission activity of a Pu239 foil to that of a U235 foil. A discussion of the ratios obtained in various assemblies which were placed in the center of a graphite thermal column is presented. The assemblies were made of natural uranium, lead, or graphite. In some cases the assemblies were surrounded by a layer of water. The experiments were conducted at thermal-column temperatures which ranged from 18 to 640°C. The data obtained in the case of the graphite assembly are used as a calibration of the neutron temperature. To within the accuracy of the experiment, the shape of this calibration curve is the same as the shape obtained from the data of C. H. Westcott. The results, for all other cases, indicate for the range of temperatures investigated that the ratio of the thermal-column temperature to the effective neutron temperature in an assembly varies linearly with the temperature of the thermal column.