A phenomenological sense of the known Hancock—Luiken separation efficiency criterion is considered in the article. Real separation operation is substituted by an equivalent scheme of parallel operations of ideal separation and ideal dis-assignation. Numerical value of Hancock—Luiken criterion for real separation operation is equal to starting feed fraction, sent to ideal separation operation in an equivalent scheme. An additional criterion is proposed, showing the cause of efficiency decrease with regard to the maximum value — concentrate contamination or valuable component losses with tailings.