The effect of alloying on the amount of plutonium required in forming a critical mass of each alloy has been expressed in terms of an inventory requirement ratio, RI. This quantity was obtained for twenty-seven potential alloying elements at three compositional levels. The effectiveness of using Vegard’s law to estimate the density of the alloys was appraised by comparing the estimated densities and RI values of nine intermetallic compounds with their x-ray densities and the RI values computed from them. The parametric variation of RI with fT, the number of excess neutrons per collision was also studied.