The treatment of wood by a mixed aceto/ oleic (or other fatty acid residue) anhydride promoted as a safe and environmentally friendly wood preservation system was examined quantitatively by liquid-phase 13C-NMR and solid-phase magic angle spinning-DEC (proton decoupling) 13C-NMR through of all its different stages to determine which reactions occurred with simple model compounds of the polymeric constituents of wood. The preparation of the mixed aceto/oleic anhydride under different conditions was also undertaken. The anhydride formed, but its percentage yield was only 30%. The mix composed of unreacted acetic anhydride, the mixed aceto/oleic anhydride, and a large proportion of free acetic and oleic acid, which are used for wood preservation, yielded the acetylation of the lignin model compound (1) by the reaction of the acetic anhydride with it and (2) by the reaction of the acetic part of the mixed anhydride. In this reaction, all of the mixed anhydride was consumed. The oleic part of the mixed anhydride was unable to form esters with either lignin or wood holocellulose as it was far less reactive than the acetic part. Some acetylation of holocellulose occurred, and some traces of its oleic acid ester also occurred under some conditions. This system of treatment through a mixed anhydride appeared to consist of just an acetylation with acetic anhydride mixed with some oleic acid as a water repellent, both of which are already known processes.