Scientists at the Los Alamos National Laboratory measured the critical masses of square prisms of highly enriched uranium diluted in various X/235U with matrix material and polyethylene. The configuration cores were 22.86 and 45.72 cm square and were reflected with 8.13-cm-thick and 10.16-cm-thick side polyethylene reflectors, respectively. The configurations had 10.16-cm-thick top and bottom polyethylene reflectors. For some configurations, the Rossi-, which is an eigenvalue characteristic for a particular configuration, was measured to establish a reactivity scale based on the degree of subcriticality. These experiments provided critical mass data in the thermal energy range for systems containing Si, Mg, Al, Gd, and Fe. The measured keff from these experiments was compared with the calculated keff from MCNP using ENDF/B-V and ENDF/B-VI cross-section data. The observed biases were +0.005 k and +0.008 k for Si, +0.0006 k and +0.008 k for Al, +0.0023 k for Mg, +0.004 k and +0.01304 k for Gd, and +0.0123 k and -0.00106 k for Fe.