Thermal tracing is a simple method for studying solids mixing in fluidized beds. However, the measurement of temperatures is influenced by both mixing and heat transfer, which limits its usefulness for inferring mixing quantitatively. In this work, a semiempirical model is developed to quantify lateral solids mixing in fluidized beds. The model couples the tracer mass balance, the enthalpy balance of tracers and bed particles, and the response dynamic of thermometers. A series of tests is pezrformed in a lab-scale fluidized bed, with particle sizes of 0.280.45, 0.450.6, 0.60.8, and 0.81.0 mm, and fluidizing velocity from 0.3 to 2.3 m/s. By evaluating the measured transient temperatures using the model, the lateral dispersion coefficient (Dsr) is determined to be between 0.0002 and 0.0024 m2/s. Its reliability is confirmed by bed collapse experiments. Finally, the values of Dsr is compared with a collection of data in the literature. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012