Flash pyrolysis oils have a high oxygen content. The presence of numerous and various oxygenated functions give these oils unwanted characteristics that may be improved by the elimination of oxygen by catalytic hydrotreating. This paper deals with the chemical characterization of RTP oils by liquid-liquid fractionation followed by mass spectrometric determination of the individual compounds. In a second part, the hydrodeoxygenation reactions were approached using model compounds selected on the basis of the characterization. The CH2Cl2 acidic fractions of the oils are mainly composed of phenolic structures. Nevertheless, the phenolic molecules are differently substituted depending on the pyrolysis conditions: carboxyl and carbonyl groups for short residence times and methoxy groups for long residence times. All the main chemical groups present in pyrolysis oils can be eliminated using nickel-molybdenum and cobalt-molybdenum traditional hydrotreating catalysts. The reactivity, the activation energies and the products of the hydrodeoxygenation conversion of the various oxygenated groups were determined.