Wattle (Acacia mearnsii De Wild., Leguminosae) bark extract is used commercially to tan leather and manufacture adhesives. The extract is treated with sodium hydrogen sulfite (sulfited) to improve its tanning properties. These include reduced viscosity, improved solubility, and better raw skin penetration. High resolution ESIMS allows unambiguous assignment of sulphur-containing monomeric and oligomeric products from sulfitation. It reveals that during sulfitation the constituent flavan-3-ol building blocks are sulfited at both C-2 and C-4 by a sulfite ion. A sulfonic acid moiety is introduced at C-2 to open the pyran ring, and C-4 to cleave the interflavanyl bond and reduce the degree of polymerization, respectively, explaining the improved tanning properties. MS2 fragmentation spectra and comparison with unsulfited extract support the interpretation of peaks and composition of sufited wattle bark extract. It also supports our published work that mimosa bark extract consists of a catechin or gallocatechin starter unit and fisetinidol or robinetinidol extender units.