The use of catalytic bed materials in fluidized bed gasifiers represents a promising primary measure to decrease the tar content of biomass-derived raw gas. For effective application of such in-bed catalysts, extensive contact must be established between the volatile matter released from the fuel particles and the bed material. However, the extent of the contact and, consequently, the potential of in-bed tar removal techniques are not well understood. In this work, the fraction of volatile matter that interacts with the bed in a large (i.e., throughput of 300-400 kg/h biomass) bubbling bed gasifier is quantified experimentally and the effect of fluidization velocity is investigated. The results show that a higher fluidization velocity enhances gas solid contact, with 48-69% of the volatile matter coming in contact with the bed within the range of 6-10 times the minimum fluidization (u(mf)).