The chemical composition of slow pyrolysis oil made from unbarked Scots pine (Pinus sylvestris) was characterized by negative-ion electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry and various bulk analyses. The wood pieces were heated stepwise in a batch pyrolysis reactor to the target temperature of 320 degrees C. The resulting pyrolysis oil spontaneously separated into aqueous and oily phases. The water content of the aqueous phase was as high as 60.4 wt %, whereas for the oily phase it was only 8.1 wt %. The bulk O/C ratio (on a dry basis) was more than twice as high for the aqueous phase as compared to the oily phase (0.61 vs 0.29), suggesting different chemistry between the two phases. On the basis of pH and total acid number measurements, the aqueous phase was more acidic than the oily phase. The ESI FT-ICR technique was used to characterize pyrolysis oils heavier, nonvolatile, and thermally labile species. The total amounts of the identified chemical formulas were similar to 540 and similar to 630 for the aqueous and oily phases, respectively. The majority of the detected compounds were oxygen-containing species, representing heteroatom classes O-2-O-14. The O-9-O-14 class compounds were present only in the aqueous phase. The O-2-O-4 compounds were comprised of different acids, including resin acids and several types of fatty acids, such as hydroxy fatty acids and diacids, which were concentrated in the oily phase. However, saturated fatty acids were more dominant in the aqueous phase, possibly due to their increased solubility through micelle formation in a more aqueous environment. Anhydrosugars and other high oxygen-containing compounds were present only in the aqueous phase. Both phases also contained condensed aromatic compounds (pyrolytic lignin), representing heteroatom classes O-3-O-8.