Experimental and theoretical studies of both vertical and lateral mixing of solids in a two-dimensional bubbling fluidized bed (1300x920x60 mm(3)) are conducted by using heated particles as the tracer. The effect of particle size on solids mixing is studied. Fluctuations in the concentration responses along both vertical and lateral directions have been observed experimentally. These fluctuations become increasingly more prominent as the bed particle size increases. The convection-diffusion model of solids mixing [L. Shen, M. Zhang, Y. Xu, Powder Technol. 84 (1995) 207-212] is used to analyze the experimental data. The dynamics of solids exchange between the bubble wake and the emulsion phase in bubbling fluidized beds is investigated, and a new model for exchange coefficient is proposed. In the model, particle size d(p), particle density rho(p), gas viscosity mu(f), gas density rho(f), minimum fluidization velocity U-mf and bubble size D-B, etc., are taken into account:where A=1.0X10(7). The exchange coefficient K-w values for the model do not compare favorably with the predictions of two available theoretical models in the literature [K. Yoshida, D. Kunii, J. Chem. Eng. Jpn. 1 (1968) 11-16; T. Chiba, H. Kobayashi, J. Chem. Eng. Jpn. 10 (1977) 206-210]. These models predict that the wake exchange coefficient should increase with increase in minimum fluidization velocity U-mf (i.e., particle size d(p)). However, our model shows that the wake exchange coefficient K-w decreases with minimum fluidization velocity U-mf. With the model of wake exchange coefficient, the convection-diffusion model of solids mixing is found to predict the experimental trends reasonably well. The model for solids exchange between the bubble wake and the emulsion phase in a gas fluidized bed is needed in modeling solids mixing and segregation patterns.