Direct determination of exposed grain surface area is a difficult task to achieve since the exposed surface area of grains in multiphase particles can only be accurately analyzed in three dimensions. Now, due to advances in high resolution X-ray microtomography (HRXMT), with a voxel resolution of similar to 1 mu m it is possible to identify dispersed grains as small as 5 mu m in multiphase particles and quantify their surface area exposure in 3D. In this paper, the development of procedures for detailed analysis to quantify the extent of grain surface area exposure using HRXMT is described. Image analysis procedures, including preprocessing of the 3D tomographic images, correction for the partial volume effect (PVE), and the use of a new algorithm for determination of exposed grain surface area measurement, are discussed. Application of the procedure is illustrated with the analysis of flotation products from HydroFloat (HF) experiments for an auriferous pyrite ore. The results demonstrate that two levels of analysis are appropriate for understanding the flotation of locked particles. First, the particle distribution with respect to exposed grain surface area, expressed as a percentage of the total particle surface area, should be considered. Second, the grain distribution with respect to actual exposed surface area should be considered in order to account for the stability of attached bubbles and recovery of such multiphase particles in the concentrate. (C) 2016 Elsevier B.V. All rights reserved.