Different mass spectrometric approaches were used toidentify an original non-terpenoid varnish. Directtemperature-resolved MS and pyrolysis−GC/MS mainlyshowed the phenolic components, whereas thermallyassisted (trans)methylation with tetramethylammoniumhydroxide (TMAH) strongly enhanced the evidence fortung oil as part of the varnish. Transethylation studiesofthe fatty acids confirmed the TMAH data. The degreeofhydrolysis of the oil network was found to be low. Noevidence was found for a direct link between the dryingoil and phenolic resin. On the basis of the MSinformation, the aged varnish is identified as an open-structuretert-butylphenol−formaldehyde resin which entanglesthetung oil polymer.