Lignin is a heterogeneous aromatic biopolymer that accounts for nearly 30% of the organic carbon on Earth(1) and is one of the few renewable sources of aromatic chemicals(2). As the most recalcitrant of the three components of lignocellulosic biomass (cellulose, hemicellulose and lignin)(3), lignin has been treated as a waste product in the pulp and paper industry, where it is burned to supply energy and recover pulping chemicals in the operation of paper mills(4). Extraction of higher value from lignin is increasingly recognized as being crucial to the economic viability of integrated biorefineries(5,6). Depolymerization is an important starting point for many lignin valorization strategies, because it could generate valuable aromatic chemicals and/or provide a source of low-molecular-mass feedstocks suitable for downstream processing(7). Commercial precedents show that certain types of lignin(lignosulphonates) may be converted into vanillin and other marketable products(8,9), but new technologies are needed to enhance the lignin value chain. The complex, irregular structure of lignin complicates chemical conversion efforts, and known depolymerization methods typically afford ill-defined products in low yields (that is, less than 10-20wt%)(2,10,11). Here we describe a method for the depolymerization of oxidized lignin under mild conditions in aqueous formic acid that results in more than 60wt% yield of low-molecular-mass aromatics. We present the discovery of this facile C-O cleavage method, its application to aspen lignin depolymerization, and mechanistic insights into the reaction. The broader implications of these results for lignin conversion and biomass refining are also considered.