Subcommittee 8 of the Standards Committee of the American Nuclear Society has proposed a standard providing subcritical limits for operations with mixed oxides of plutonium and uranium. The subcritical limit is the limiting value assigned to a controlled parameter that results in a system known to be subcritical, provided the limiting value of no other controlled parameter of the system is violated. The proposed standard includes subcritical limits for mixed oxides containing up to 30 wt% plutonium in Pu + U. A review was made of the available experimental data and validations undertaken that serve as the basis of the limits, and the assertion that they are, indeed, subcritical as given.