Alkaline cupric oxide oxidation and proximate analysis were used to investigate the sources and diagenetic state of organic matter in sixSphagnum-dominated peatlands located between Alberta, Canada, and Ohio, U.S.A. Cupric oxide oxidation was also used to characterize vascular and nonvascular wetland plant species to provide a specific biological fingerprint of these plant tissues. Oxidation of 15 species ofSphagnummoss released large quantities of unsubstitutedp-hydroxyl phenolic compounds as well as the species specific sphagnum acid (p-hydroxy-&bgr;-[carboxymethyl]-cinnamic acid). By contrast, vascular plant tissues released large amounts of lignin oxidation products. Cupric oxide oxidation ofSphagnumpeat from more northerly sites produced mainlyp-hydroxyl phenolic monomers with lesser amounts of vascular lignin derived phenols. In contrast, southern sites and those dominated by woody vegetation produced oxidation products characteristic of vascular plant lignin. A distinct relationship exists between the amount of acid-insoluble Klason lignin and both the diagenetically sensitive phenolic acid to aldehyde ratios as well as the total yield of vanillyl phenolic oxidation products. We found evidence of selective decay of phenolic lignin precursors. These relationships indicate the lignin component in surficial layers ofSphagnum-dominated peat is influenced by bothSphagnumand vascular plant lignin, and the structure of lignin appears to undergo diagenetic changes in these layers. Application of an end-member mixing model revealed that lignin oxidation products poorly predicted vegetational composition of the lignin in more decomposed peat, probably as a result of selective decay of lignin structural phenols.Key words: lignin, organic soil, proximate analysis,Sphagnummoss, wetland.