Fluidized beds are particularly favored as chemical reactors because of their ability to exchange heat through immersed heat-exchange surfaces. However, little is known about how the heat-exchange process works on a single-particle level. The most commonly applied theory of fluidized bed heat exchange is that developed by Mickley and Fairbanks in the 1950s-the so-called packet model. The work described in this article is an attempt to understand the process of heat transfer by solids convection, using positron emission particle tracking to follow the trajectory of a single tracer particle in the bed. In particular, the residence time of particles in the vicinity of the surface is determined here for the first time. Using these data, the observed heat-transfer variations are interpreted mechanistically. (c) 2006 American Institute of Chemical Engineers.