A thermal technique has been developed for measurement of solid mixing in fluidized beds. A batch of bed particles is preheated and then injected into the bed. An array of thermistor probes measures the spread of the tracer. A series of experiments has been done in the upper core region of a 7 m tall, 0.20 m diameter test circulating fluidized bed (CFB). The experiments involved one type of particle, having 180 mum diameter and 2350 kg/m3 density. Models for the spread of tracer particles, transfer of heat from tracer particles to surrounding gas and particles, and for heat transfer to the thermistors were used to make estimates of the radial particle diffusivity in the core region. The diffusivity was on the order of 10 cm2/s for the range of velocities and solid concentrations tested. A second set of preliminary solid mixing experiments has been done in the bottom of a 1.8 m tall, 0.16 m square CFB using 55 mum FCC (fluid cracking catalyst) particles. The mixing in this region, although not representative of a strictly diffusional process, appears to be somewhat more vigorous than the core mixing in the upper region of the bed.