Gas-solid interaction does exist both outside and inside particle clusters in gas-solid two-phase flow, but the penetrating flow through clusters was generally neglected in most mathematical models of clusters. As regards to what extent the penetrating flow exists in gas-solid flow and how the penetrating flow affects gas-solid interaction, there are still no universally accepted viewpoint and quantitative characterization until now. By utilizing both CFD (Computational Fluid Dynamics) simulations and PIV (Particle Image Velocimetry) measurements, this article quantitatively investigates the gas and dilute gas-solid penetrating flows through different pseudo two-dimensional clusters in a two-dimensional gas-solid fluidized bed. The solid penetrating flow through clusters seldom occurs, but the gas penetrating flow increases exponentially with increasing internal voidage of clusters, which is however affected little by superficial gas velocity under the tested operating conditions. These results imply that the effect of the gas penetrating flow through clusters must be taken into consideration when investigating particle clustering behavior in gas-solid two-phase flow. (C) 2012 Elsevier B.V. All rights reserved.