The objective of this study is to provide background information on pyrolysis oil for gas turbine applications. The bio-oil investigated consists of an upper layer and a bottom layer. It has been produced by vacuum pyrolysis of softwood bark. The storage stability and the thermal behavior of the whole bio-oil, the upper layer, and the bottom layer were evaluated. The samples were stored at different temperatures (40, 50, and 80 degreesC) for up to 168 h and at room temperature for up to 1 year. Morphology, rheology, thermogravimetry, and differential scanning calorimetry tests were performed on the bio-oil samples after their aging. The results indicated that the properties of the whole bio-oil are significantly altered when the bio-oil was heated at 80 degreesC, whatever the range of time investigated. The impact after heating to 50 degreesC was not critical for storage and handling purposes however, as shown by a series of rheological data. It was found that the aging effect is more pronounced for the bottom layer than for the upper layer. The presence of the upper layer reduced the aging rate of the bio-oil as a whole. The molecular weight increase after heating the whole bio-oil for 1 week at 80 degreesC was equivalent to keeping the sample for 1 year at room temperature. This increase was mainly caused by polymerization of the bottom-layer-contained compounds. Aging of the raw bio-oil at room temperature resulted in a dramatic viscosity increase during the first 60 days, after which a plateau was reached.