A wide range of industrial operations involve the mixing of multiple phases, where the interfacial area between the phases determines the performance of the system. Interactions between phases occur, giving rise to the formation of complex structures containing, for example, air bubbles and small water droplets, trapped in the organic/oil drops. A two-dimensional observation of this phenomenon may lead to an erroneous determination of the amount of dispersed elements and the actual interaction between them, since bubbles and drops coming from different focal planes may appear overlapped. In this work, a method that allows the accurate determination of embedded particles within dispersed elements was developed and validated. By using micro-stereoscopic on-line image acquisition techniques, accurate images from a four-phase system model simulating a fermentation broth were acquired to identify and count the oil-trapped particles moving at high speed. Light refraction effects within embedded particles were considered to obtain highest accuracy. The total error of the measurement system was mainly due to the magnification used for the stereo-microscope and the size range of particles to be measured, being as low as +/- 5.7% for x6 magnification and particles as small as 250 pm. Moreover, 3D results clearly indicated that bi-dimensional observations lead to an erroneous evaluation of the extent of the particles trapped (overestimated by up to 30%) since different focal planes may be overlapped. This original method permits to postulate very accurate models for mass transfer, based on experimental evidence of the actual multiphase systems. (C) 2007 Elsevier Ltd. All rights reserved.