Critical experiments were conducted with homogeneous mixtures of PuO2 - polystyrene (H:Pu atomic ratio of 15) containing both 2.2 and 8.0% Pu240. Criticality was determined for a series of Plexiglas reflected rectangular prisms ranging from near cubes, to long columns, and to thin slabs; bare arrays of near-cubic geometry were also studied. Critical thicknesses were 16.09 ± 0.41 and 5.99 ± 0.10 cm, respectively, for the bare and reflected infinite slabs of PuO2-polystyrene containing 2.2% Pu240. Corresponding values for the 8.0% Pu240 mixtures were 18.48 ± 0.41 and 7.38 ± 0.09 cm. The infinite slab thicknesses for an equivalent Pu239-water mixture (H:Pu = 15, ρ = 1.62 g Pu/cm3) were 11.66 ± 0.30 and 4.38 ± 0.08 cm, respectively, for the bare and water-reflected slabs. Corresponding critical radii for infinitely long cylinders were 10.52 ± 0.16 and 6.54 ± 0.14 cm; radii for critical spheres were 13.81 ± 0.16 and 10.40 ± 0.17 cm.