Abstract Phytochemical extractives can include hundreds of molecules in diverse classes—including soluble components such as flavonoids, proanthocyanidins, and terpenes—and they make-up several percent of the non-carbohydrate dry mass in small diameter Douglas-fir forestry residues. We establish that aqueous extracts from the most extractive rich tissue of Douglas-fir, bark, are inhibitory at biofuel relevant concentrations and demonstrate the inhibition with initial rate saccharification experiments using Trichoderma reesei cellulases. Initial rates of carbohydrate production from crystalline cellulose and amorphous cellulose substrates suggest that exoglucanase, endoglucanase and β-glucosidases are each inhibited between 15 and 25% in the presence of bark extracts. We find that despite the chemical complexity of this feedstock, the bulk of the inhibition comes from tannins while other suspicious extractive components we tested produced no statistically significant inhibition (in conditions of up to 3 grams per liter dry extractives).