Plant material is the most abundant biomass on earth, consisting mainly of cellulose, xylan and lignin. Biodegradation of this lignocellulosic material is critical for carbon cycling. Xylophagous invertebrates play a significant role in maintaining the carbon balance in nature. While mechanisms of cellulose digestion in insects are being elucidated, understanding degradation of lignin and aromatics have remained incomplete and have yet to be clearly demonstrated in a natural system. Because of its non-repetitive subunit structure and cross-linkage with plant cell walls, lignin is difficult to degrade and can significantly impede access to carbohydrates in cellulose. Using C-tetramethyl-ammonium hydroxide (TMAH) thermochemolysis followed by GC-MS, we present convincing evidence of marked lignin degradation in the gut of the Asian longhorned beetle (ALB, Anoplophora glabripennis) and the Pacific dampwood termite (Zootermopsis angusticollis) by examining the frass excreted by these insects. ALB completes its lifecycle deep in the wood of a wide range of living hardwood trees, while Z. angusticollis feeds in dead trees. Lignin degradation was measured by comparing the profile of TMAH thermochemolysis products in frass with undigested wood from host tree tissue. The frass from both insects contained an increase in the ratio of acid to aldehyde structures, indicating lignin degradation. While wood-feeding termites are known to harbor microbes in their gut that likely play a role in lignin degradation, until now there was little evidence that directly showed degradation of polymeric lignin in the gut of any insect species. This demonstration of lignin modification in two distantly-related insect species suggests that the ability of xylophagous insects to degrade lignin may be more common than previously thought, playing a valuable role in carbon and nitrogen cycling in nature.