A new and simple protocol for quantitative analysis of bio-oils using gas-chromatography/mass spectrometry is suggested. Compounds were identified via their mass spectra, and then unavailable response factors were calculated with respect to diphenyl as the internal standard using a modified method previously suggested for gas chromatography with flame ionization detection. This new protocol was applied to the characterization of bio-oils obtained from the pyrolysis of woods of different sources or using different pyrolysis procedures. This protocol allowed evaluation of the yields of products from poplar pyrolysis (among 50% and 99%), while a reduced amounts of products were identified from the pyrolysis of cellulose (between 46% and 58%). The main product was always acetic acid, but it was formed in very large yields from poplar while lower yields were obtained from cellulose.