Comparisons of calculated and measured neutron balance components are reported for the 7.5% fissile plutonium reference test lattice of the PROTEUS—light water high conversion reactor (LWHCR) phase II program, both wet (with H2O) and dry (100% void). Special experimental techniques have been developed and applied, particularly for k∞, and the range of directly measured reaction rate ratios has been extended. For the two cell codes tested, WIMS-D/1981 library and KARBUS/KEDAK-4, specific shortcomings have been identified; the new measurements have been found to be significantly more representative and accurate than the earlier phase I experiments. The k∞, void coefficient for the phase II reference lattice between 0 and 100% void has been found to be qualitatively different from those assessed for the earlier phase I test lattices. Consideration of the individual void coefficient components show this to be largely a consequence of the more LWHCR-representative fuel rod diameter and plutonium isotopic composition of the fuel currently being used. Results of control rod studies conducted for the phase II reference lattice—both wet and dry—serve to illustrate the efforts being made toward investigations of special power reactor features.