Summary: There is an important corollary to Prior's rule to the effect that as the state of oxidation of meteorites is increased the quantity of olivine increases relative to pyroxene and metallic iron. We have derived a rigorous parametric expression for this corollary and find that, as in the case of the quantitative interpretation of the rule itself, there is in this case also a fair correspondence to the observational data, so that the validity of the rule is further supported.