The behaviour of olivine compared to silica sand as bed material was investigated in a dual fluidised bed reactor system (DFB) for steam gasification of biomass. This study presents fundamental experimental data on the impact of olivine on the product gas and distribution of condensable hydrocarbons (tars) effected by the catalytic properties of olivine. The results obtained with olivine are compared to silica sand, which is taken as a reference as it is considered to be inert. The experimental device is a pilot plant of 100 kW fuel input and the design is comparable to large scale applications. Pressure and temperature profiles of the reactor system are presented to illustrate the system performance. A clear shift was found for the product gas composition (H(2): 5 percentage points in volume, CO and CO(2): up to 10 percentage points in volume) using silica sand and olivine as bed material. Olivine is identified as increasing the product gas yield and the H(2) yield by promoting the CO-shift reaction. It was found that olivine reduced the GC/MS detectable tar content by approx. 35% and the content of gravimetrically detected tars by approx. 60% in the product gas. The changes in tar yield (related to the fuel input) were found to be decreased by about 30% and 57% for GC/MS and gray, detectable tars, respectively. Classifications for condensable hydrocarbons proposed in literature are used to lump the detected tar species into appropriate groups. Thus, the behaviour and shifting of the tar with respect to the classification is identified. In particular, naphthalene was found to contribute notably to the GC/MS tar complex at quantities of 40-45 wt.% (olivine) and 31-33 wt.% (silica sand). In contrast to silica sand, olivine has a decomposition effect on the higher hydrocarbons. (C) 2011 Elsevier BM. All rights reserved.