Calculations of the infinite multiplication factor for aqueous homogeneous mixtures of mixed oxides of plutonium and natural uranium at low fissile concentrations (7 g Pu/ℓ ) disclose a maximum to occur in the value of k∞ at a weight fraction, Pu/(Pu + U), of ∼0.0035. With mixed oxide solutions containing 7 g Pu/ℓ, the value of k∞ is estimated to be nearly 1.04, whereas in the absence of the natural uranium, the maximum value of k∞ at 7 g Pu/ℓ in water is ∼4% less or near unity. The occurrence of this peak in value of k∞ is due to the 235U content in the natural uranium. Thus, in the presence of natural uranium, it should be borne in mind that the limiting subcritical concentration of plutonium (given as 7.0 g Pu/ℓ) in water must be reduced to values <7.0 g Pu/ℓ to ensure sub criticality of the mixture.