A series of experimental measurements has been made on the Experimental Gas Cooled Reactor (EGCR) lattice in the Physical Constants Test Reactor (PCTR). The measurements provide a broad basis for normalization of reactor calculations for lattices of this type. The fuel assembly is a cluster of seven uranium oxide rods, enriched in the U235 isotope and clad with stainless steel. The fuel is spaced on an eight-inch square pitch in a graphite moderator. Values of the lattice parameters k∝ , f, p, and e have been obtained for 1.8% enrichment of the uranium oxide fuel. The values of k∝ and f have also been obtained for 2.6% enrichment fuel. The techniques of using the PCTR have been extended so that supercell measurements may be made. The values of the strength of a boron carbide control rod and a stainless steel loop tube have been obtained in this way. The strength of such an inhomogeneous poison in the lattice is expressed as the difference in the supercell multiplication factor k∝ with and without the poison in the supercell. This difference is the same quantity which is obtained in the usual reactor cell calculation. The fuel temperature coefficient of for this cluster has also been measured between 50 and 500°C. The coefficient obtained is temperature dependent. The more important of the lattice parameters for the 1.8% enriched fuel are = 1.146 ± 0.004,f = 0.809 ± 0.005, p28 = 0.824 ± 0.006, ∈ = 1.019 ± 0.002, Δk (control rod -16 cell supercell) = -0.157 ± 0.012, Δk (empty loop tube -9 cell supercell) = -0.117 ± 0.011, and (l/k∞)(dk∞/dT) = -(0.68 ± 0.05) X 10-3T-1/2(oK)-1 For the 2.6% enriched fuel, results are k∞ = 1.256 ± 0.009 and f = 0.845 ± 0.006.