The availability of experimental information on particle distribution inside stirred tanks is a topic of great importance for many industrial applications such as catalysis and polymerization. The measurement of solid particle distribution is not simple and the development of suitable measuring techniques is still an open problem in chemical engineering research. In this work, a non-intrusive optical technique for measuring particle concentration fields in solid-liquid systems is discussed. The "Laser Sheet Image Analysis" (LSIA) technique described here makes use of a laser sheet, a digital camera for image acquisition and a suitable procedure for post-processing. It is able to provide solid concentration distribution maps on the laser sheet plane. The technique developed is applied to the case of an unbaffled vessel stirred by a Rushton turbine and provided with a top cover in order to avoid vortex formation. Visual observation of the system reveals that particles tend to concentrate in two tori located above and below the impeller plane, respectively. The phenomenon is more marked with heavy (either larger or denser) particles, that practically concentrate exclusively in the two tori, leaving the rest of the vessel substantially free from particles. These features are fully confirmed, as well as quantitatively assessed, by the results obtained via the LSIA technique. (C) 2012 Elsevier Ltd. All rights reserved.