The High Temperature Lattice Test Reactor (HTLTR) is a unique critical facility specifically built for studying reactor lattices at temperatures up to 1000°C. A description of the reactor is given and the unpoisoned technique for determining >, the neutron multiplication factor, from the ratio of the reactivity coefficients of a test sample of a neutron multiplying medium and a normalizing thermal neutron absorber is discussed. Comparisons of <img src= as measured by the unpoisoned method and the well known Physical Constants Testing Reactor null reactivity method method are given for a natural UO2-2.2 wt% PuO2 (8% 240Pu) graphite lattice and for a UO2 (97.42 wt% 233U)-232ThO2-C fuel element in a graphite array. Experimental values for the neutron multiplication factor have been determined as a function of temperature using the HTLTR for a graphite moderated lattice fueled with a 235UC2 -232ThO2 -C fuel mixture. These results provide valuable check points for testing computational methods currently being applied in the design of high temperature gas cooled reactors.