One of the most crucial steps in the development of fundamental hydrodynamic models is the validation of these models with accurate, detailed experimental data. Therefore a whole-field, non-intrusive digital image analysis technique has been developed which enables measurement of bed expansion and segregation dynamics of coloured particles in dense gas-fluidised beds. The development, calibration and accuracy of the technique are discussed in detail. The image analysis technique traces bubbles and voidage waves accurately, whereas the mixture composition in a fluidised bed could be determined within 10%.Experiments have been carried out with 1.5 and 2.5 mm coloured glass beads, for which particle-particle and particle-wall collision parameters were accurately known. They were performed in pseudo two-dimensional laboratory scale fluidised beds with a simple rectangular geometry and well-defined gas inflow conditions. An extensive set of results obtained with both mono-disperse systems and binary mixtures, suitable for validation of fundamental hydrodynamic models, is presented. (C) 2003 Elsevier B.V. All rights reserved.