Effect of particle acceleration/deceleration on particle clustering behavior in dilute gas-solid flow was studied by experimental measurements and mathematical modeling. Calculation results of the model are in good agreement with experimental data from Phase Doppler Particle Analyzer (PDPA) measurements. The variation of voidage inside particle clusters is strongly dependent upon the change of the number of particles within the clusters. During acceleration, particle clusters are gradually disaggregated into smaller clusters with increasing voidage, while during deceleration, particle clusters are aggregated into larger clusters with decreasing voidage. (c) 2006 Elsevier Ltd. All rights reserved.