Residence time in a single flotation machine is one of the most important parameters necessary to properly design a flotation circuit. The mean residence time for any particular cell design is usually determined from residence time distribution (RTD) experimental results. The comparison of the measured mean residence time with the expected residence time, as well as analysis of the shape of the RTD curve, give the most useful and valid information about the mixing properties in the cell. Experimental RTD results for three large flotation cells (Dorr-Oliver, Outokumpu and WEMCO(R)) discussed in this publication were released by the Chuquicamata Division of CODELCO as a part of the competitive rougher flotation test report (CODELCO (National Copper Corporation of Chile), Chuquicamata Division, 2000, Project IM2, No. 19/99; Final Report of the Evaluation of Large Volume Flotation Cells). Evaluation of the results by CODELCO as well as by authors is provided. "Approximation of the theoretical curve of a perfect mixer," used by CODELCO as a base for cell categorization is analyzed and critiqued after comparing to the criteria used by many authorities in the field. Conclusions are drawn based on the multi-parameter flow model, showing existence of stagnant volume of 45% in Outokumpu, 35% in Dorr-Oliver, and 7% in WEMCO(R) flotation cell volumes. (C) 2002 Elsevier Science Ltd. All rights reserved.