An arithmetic approach was taken to the accounting of mass fractions and chemical components during the fractionation of red oak (Quercus I ubi a) wood chips into fibers and polymer products representing biomass constituents. The fractionation involved sequential treatments by steam explosion, water washing, and lignin (incl. lignin-like extractives, esp. tannins and carbohydrate degradation products, esp. furans)-extraction with either aqueous alkali, aqueous acetic acid, or aqueous ethanol. Cleanness of fractionation was assessed in terms of the release of constitutive component mass from a solids fraction during each processing step. Fractionation behavior was studied in relation to different steam treatment severities. The results indicate that best fractionation conditions for hemicelluloses are reached at low treatment severities (R-o ca. 10,000) where up to 72% of this component may be collected in the water extract; and that best lignin/tannin yields are achieved at high treatment severities (R-o ca. 35,000) where 82% of theory may be harvested in the alkali-soluble fraction. Best results require the elimination of all fiber losses during steam explosion, a condition not reached in the present study. The fractionation performance in either organic solvent mixture, 80% acetic acid or 70% ethanol resulted in 20% less delignification. Lignin is progressively contaminated with associated furfural as severity increases. Above a severity of R-o 10,000, water extracted fiber solids are virtually hemicellulose-free.