A photometric technique was used to determine the apparent concentration of particles inside voids in two-dimensional fluidized beds of fluid catalytic cracking particles. Light was transmitted when voids passed in front of a laser beam, with attenuation occurring due to dispersed particles in the path of the beam. The results show that more particles are dispersed inside the voids at higher superficial gas velocities and that a broader particle size distribution leads to more particles inside the dilute phase. For the group A powders investigated, approximately 4% of the volume of detectable voids was typically occupied by particles, this being about an order of magnitude higher than earlier experimental estimates obtained with group B powders.