As part of the Yankee and the Belgian Reactor-3 Critical Experiments program at the Westinghouse Reactor Evaluation Center, measurements have been made of the parameters leading to p, f, and ϵ in a light-water moderated heterogeneous reactor with slightly enriched UO2 fuel rods clad in stainless steel. A detailed description of this reactor is given in reference 1. Measurements were made using 4.48% and 2.73% enriched fuel with lattice pitches of 0.470 and 0.435 in. The individual UO2 sintered pellets, in the fuel rods, had a diameter of 0.300 in. and a length of 0.600 in. The 4.48% and 2.73% fuel rods contained 90 and 80 pellets, respectively. The stainless steel cladding was 0.305 in. i.d. and 0.347 in. o.d. for the 4.48% enriched fuel, and 0.306 in. i.d. and 0.338 in. o.d. in the case of 2.73% enrichment. The measurements were performed using a higher fuel enrichment than used previously at Bettis (2, 3) and elsewhere for similar experiments, thus uncovering a large unexplored range of enrichments.