Critical masses of 6-in. diameter cylinders of plutonium diluted in various volume ratios with steel, aluminum, thorium, air, and depleted uranium were determined. The cores were in 0, 2, 4.5, or 7.5-in. thick uranium reflectors and in 2, 4.5, or 7.5-in. thick thorium reflectors. From these data, the equivalent spherical critical masses of plutonium have been estimated. An appendix gives reflector savings of undiluted plutonium for a variety of materials.