A styrene-butadiene-styrene (SBS) block copolymer has been used as a compatibilizer in a low density polyethylene/wood flour (LDPE/WF) composite system. Dynamic mechanical thermal analysis (DMTA) and a transmission electron microscopy (TEM) were used to investigate the interfacial properties in the composites. A high resolution study of the composite microstructure, and especially of the interfaces between the wood particles and LDPE matrix, indicated that the SBS compatibilizer was located at the interface region between the wood particles and LDPE matrix and partially covered the wood particle surfaces. The SBS was also found in the LDPE matrix. The unsaturated part of the copolymer was stained with osmium(VIII)tetraoxide (OsO4) to enhance contrast and to allow it to be detectable in TEM. Dynamic mechanical measurements confirmed interaction between polystyrene (PS) and wood in the PE/PS/WF system. The tan d peak of PS was shifted about 10°C to a higher temperature and also broadened when wood flour was added in the LDPE/PS blend.