In this study, the application of response surface methodology (RSM) and central composite rotatable design (CCRD) for modeling the influence of some operating variables on the performance of a Multi-Gravity Separator (MGS) for coal cleaning was discussed. Four operating variables of MGS, namely drum speed, tilt angle, wash water and feed solids were changed during the tests based on the CCRD. In order to produce clean coal with MGS, mathematical model equations were derived by computer simulation programming applying least squares method using MATLAB 7.1. These equations that are second-order response functions representing ash content and combustible recovery of clean coal were expressed as functions of four operating parameters of MGS. Predicted values were found to be in good agreement with experimental values (R-2 values of 0.84 and 0.93 for ash content and combustible recovery of clean coal, respectively). This study has shown that the CCRD and RSM could efficiently be applied for the modeling of MGS for coal and it is economical way of obtaining the maximum amount of information in a short period of time and with the fewest number of experiments. (c) 2006 Elsevier Ltd. All rights reserved.