The multiphase complex structure of biomass pyrolysis oils can be attributed to the presence of char particles, waxy materials, aqueous droplets, droplets of different nature, and micelles formed of heavy compounds in a matrix of hollocellulose-derived compounds and water. Bio-oil complexity is illustrated by use of two oils produced from the vacuum pyrolysis of softwood bark residues (SWBR) and hardwood rich in fibers (HWRF). The effect of waxy materials on bio-oil behavior was Studied by differential scanning calorimetry (DSC), optical microscopy, and surface tension measurements as well as by steady and dynamic rheological techniques. At around 45 degrees C the melting of waxy materials occurs. Bio-oil cooling rate was found to have an important effect on some bio-oil theological characteristics. This may be due to crystallization of waxy materials. Strain and frequency sweep tests proved the existence of a network (gel) structure in the bottom layer of oils derived from SWBR. This structure may be formed of relatively heavy oligomeric compounds that are associated in form of micelles. The observed network structure disappears above 60 degrees C. The HWRF-derived oil behaves much more like a Newtonian fluid.