The separation performance of a Floatex Density Separator (FDS) for Indian coals has been studied. Experimental results reveal that both density as well as size have substantial influence on separation performance. It is seen that at a lower bed pressure the unit acts more like a classifier rather than a concentrator. With increasing bed pressure, the classification effect is, however, reduced and the concentration effect becomes more dominant due to proper development of suspension density. The separation performance of the FDS is described with density as well as size partitions. It is found that the size or density alone is not able to describe the separation fully. The terminal settling velocity, which includes both the size and density of a particle, can be a useful descriptor of separation in FDS. U-t50 may be defined as the terminal settling velocity of those particles that have equal tendency to report to the overflow or to the underflow stream. As a result, a higher teeter water flow rate helps in carrying larger-lighter particles to the overflow (higher settling velocity) by increasing the cut off terminal settling velocity as it increases the net upward force. (C) 2007 Elsevier B.V. All rights reserved.