The mass transfer coefficient around freely moving active particles under bubbling/slugging fluidized bed conditions was measured in a lab-scale reactor. The technique used for the measurements consisted in the oxidation reaction of carbon monoxide at 450 degrees C over one or few Pt catalyst spheres immersed in an inert bed of sand. It was shown that this technique is simple and accurate, and allows to overcome most of the difficulties and uncertainties associated with other available techniques. The experimental campaign was carried out by varying the fluidization velocity (0.15-0.90 m/s), the active particle size (1.0-10.0 mm) and the inert particle size (0.1-1.4 mm). Results were analyzed in terms of the particle Sherwood number. Experimental data showed that Sh is not influenced by the fluidization velocity and by a change of regime from bubbling to slugging, whereas it increases with a square root dependence with the minimum fluidization velocity and with the active particle size. These results strongly suggest that the active particles only reside in the dense phase and never enter the bubble/slug phase. Data were excellently fitted by a Frossling-type correlation: Sh = 2.0 . epsilon(mf) + K . (Re-mf/epsilon(mf))(1/2). Sc-1/3 with a constant K = 0.70. All the other empirical/theoretical correlations available to date in the literature failed in predicting our experimental data, except for the purely empirical correlation by Prins et al. [1985. Mass transfer from a freely moving single sphere to the dense phase of a gas fluidized bed of inert particles. Chemical Engineering Science 40, 481-497], but only for an inert particle size below 700 mu m. (c) 2007 Elsevier Ltd. All rights reserved.