The evaluation of the heat transfer coefficient h(wp) between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis.The inclusion of h(wp),, or an equivalent formation, in mechanistic models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate h(wp).A precise formulation for h(wp) accounting for transient conduction inside Spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (N-p) and an effective separation gap (l(0)) is developed here.Although N-p can be estimated, in principle, from experimental evidence in packed beds, and it is reasonably expected that l(0) = 0, the analysis of experimental heat transfer rates in moving beds, packed beds, and bubbling fluidized beds indicate that values of h(wp) are, in general, smaller than expected from these assumptions. Appropriate values of l(0) and N-p are then estimated by fitting the experimental data.The probable effect of surface asperities is also discussed by analyzing a simplified geometrical model. It is concluded that the parameter l(0) can be also effective to account for particle roughness, independently of thermal properties.