This study proposes a deinking selectivity concept that considers both ink removal and fiber yield in determining the performance of deinking operations. The defined deinking selectivity, or Z-factor, is expressed by the ratio of ink removal expressed by the International Standards Organization (ISO) brightness gain or the reduction in relative effective residual ink concentration (ERIC) and the relative fiber (oven-dry basis) rejection loss. For most flotation processes, typical brightness Z-factor is on the order of unit value and ERIC Z-factor is on the order of 10 units. Therefore, the Z-factor weighted brightness gain and ERIC reduction have relevance to ISO brightness and ordinary ERIC reduction. Pilot-scale flotation deinking experiments showed that Z-factor weighted brightness gain and ERIC reduction are good indicators of deinking process efficiency. The period or stage Z-factors are good indicators of the efficiency of the periods or stages of a deinking process. The ERIC Z-factor can be directly related to the separation factor defined in chemical engineering science. A simple criterion developed using the stage Z-factor concept was applied to both pilot-scale experiments and an industrial recycling mill operation for determining the economics of a given period or stage in a flotation deinking operation. (C) 2004 Elsevier B.V. All rights reserved.