Phenolic compounds with antioxidant activity toward soybean-derived fatty acid methyl esters (SME, used as a model for biodiesel) were obtained by pyrolysis from wood (birch hardwood), corn stover, and lignin. The lignins were either commercially available (Indulin AT kraft lignin from softwood) or isolated from whole plant material by acid-dioxane extraction. Phenols were isolated from the crude liquid pyrolyzate by extraction with alkali. Antioxidant activities were determined by ferric isothiocyanate spectrometry, Rancimat induction period determination, and pressurized differential scanning calorimetry. Preliminary evidence with hydrocarbon diesel-SME mixtures indicated that the mixtures differed from SME alone in their response to antioxidants. The total phenolic extracts from birch wood pyrolysis and kraft lignin were active antioxidants in all three analyses, comparable in effectiveness to the synthetic antioxidant BHT. However, many simple lignin-related phenolic monomers were found to have little or no activity when tested with SME. Chromatographic separation and analysis of the extracts indicated that the bulk of the activity was associated with a more complex fraction, made up principally of dimers in the size range of lignans, with molecular weights from 272 to 344. A model lignan, the naturally occurring nordihydroguaiaretic acid (molecular weight 302), was shown to have very good effectiveness, as good as or better than BHT. (However, it is not likely to be present in our extracts, based on its mass spectral characteristics.) A suggested mechanism for antioxidant effectiveness of lignin-like dimers was proposed based on steric effects – simultaneous hydrogen bonding and radical quenching.