In attempting to model the process of grinding inside a fluid energy mill, it was found necessary to characterise the centrifugal classification inside the classifying chamber of the mill. By suitably modifying the circular mill, provisions were made to collect the product, as well as the recycled material, so that the material balance could be taken at the point of classification. The analysis was made based on the classification function Ci, defined as the fraction of the material of size i recycled to the grinding chamber after classification. Experiments were carried out using a mixed feed (calcite) of all sizes less than 500 μm, for different feed rates and suction conditions. The parameter values were found to increase with particle size and feed rate, and for the same feed rate the Ci values were relatively higher for higher suction conditions. However, it was noticed during the actual grinding operation that the level of suction decreased with the feed rate. Expressing this variation with an exponential relation, the net effect of feed rate on Ci values was analysed, and it was found that the parameter values decreased with the feed rate. The results were used to define a two-parameter model for the classification process expressing Ci as a function of particle size, and the polynomial functions relating the characteristic coefficients, λ and d50, with feed rates were developed. © 1991.