Quality of fluidization is an important criterion for efficient heat and mass transfer in gas-solid fluidized beds. Knowledge of voidage distribution in fluidized bed is essential when exothermic reactions are carried out in fluidized beds. Quality of fluidization is often expressed in terms of time average local void-faction distribution. Gamma ray transmission technique is one of the techniques used to study the quality of fluidization. In the present study. Gamma ray transmission technique was explored to study the quality of fluidization in a unary fluidized bed and packed fluidized bed, in which small particles are fluidized in the interstitial void space of relatively large and stationary pebbles. Gamma ray photons were allowed to pass through a unary fluidized bed of silica and lithium-titanate particles at different angular and axial positions of the bed. Experiments were conducted in packed-fluidized bed of silica particles in the interstices of large size alumina pebbles and lithium-titanate particles in the interstitial void space of large size lithium titanate pebbles. It was observed, that packed fluidized bed provides better quality of fluidization or more homogeneous fluidization compared to that of a unary fluidized bed. Operating principle, experimental details and results are discussed in this paper. (C) 2012 Elsevier B.V. All rights reserved.