Biomass pyrolysis liquids are complex mixtures. The identification of the individual components requires the use of hyphenated techniques. We describe a new experimental procedure based on the computer-aided analysis of the carbon-13 NMR spectrum of the mixture. This procedure was checked with several artificial mixtures of phenolic derivatives (alkylphenols and substituted guaiacols). We applied this technology to identify the most abundant phenolic derivatives present in the acidic fraction of an authentic bio-oil (guaiacol, 2,6-dimethoxyphenol, ethylphenol and several derivatives).