The thermal stability of bio-oils obtained from softwood bark (SWBR) and hardwood rich in fibers (HWRF) residues was studied under aging conditions at 80 degrees C in sealed glass bottles in the presence of stainless steel and copper. SWBR-derived bio-oil separated into oily and aqueous phases during aging, while HWRF-derived oil remained as a single phase during the entire aging period. Dynamic theological tests and GPC analyses were used to follow the effect of aging on the properties of bio-oils. Bio-oil aging was determined following the changes in bio-oil macrofractions using solvents of different polarities. The main difference observed between the oils analyzed was the tendency of SWBR to form materials insoluble in methanol, while HWRF aging reactions led to the formation of compounds insoluble in water and CH2Cl2. Methanol-insoluble and water-/CH2Cl2-insoluble fractions of aged oils were characterized by thermogravimetry, GPC, and Py-GC/MS techniques. The bio-oil aging reactions seem not to be affected by the presence of metals.