The effective temperature of the thermal neutrons at the center of the internal column of the Thermal Test Reactor has been measured by means of a danger coefficient technique. Assuming that the thermal neutrons have a Maxwellian velocity distribution, the experiments indicated that the TTR neutrons have a Kelvin temperature of 300°. The unique relation which exists between a 1/v and a non 1/v neutron absorber for neutron capture as a function of energy is the underlying principle upon which the experimental technique is based. The physical temperature of the reactor during the period of experimentation was also measured as 300°K.