The criterion for efficiency for separation of binary isotopic mixtures is the well-known value function. However, in the case of multi-isotope separation. this value function does nor exist. In this paper we develop a criterion to define the efficiency Fur separation of multi-isotope mixtures. It is based on che concept of the match-abundance ratio cascade (MARC) originally introduced by De La Garza for a ternary isotope mixture The criterion has the property that, when applied to binary mixtures, it is the same as the value function. This approach has demonstrated that for obtaining the optimal parameters of a single stage in the cascade it is necessary to minimize the linear combination of the inverse values of "partial separative powers" for all mixture components. A numerical example using this efficiency criterion is presented using the separation of chromium isotopes by a single gas centrifuge.