The fractionation of two biomass resources, red oak (Quercus rubra) chips and oil palm (Elaeis guineensis) trunk solids, into constitutive chemical components, cellulose, hemicelluloses (called "other carbohydrates") and non-carbohydrates (includes lignin, tannins, etc.), was studied quantitatively in terms of relative cleanness. Red oak chips were steam exploded using a batch reactor at five different treatment severities, Ro 5,000, 10,000, 15,000, 20,000 and 35,000. Steam exploded fibers (SEF) of each severity were extracted with water and alkali. Mass fractionation and summative analysis data of all solid biomass fractions were determined. These data were interpreted in term of a unifying clean fractionation concept designed to evaluate the effectiveness of the fractionation processes. Within a series of severities applied to a single biomass resource, the quantitative clean fractionation can be used to choose an optimum severity for the isolation of any particular component fraction. The red oak results revealed that 25 % (on average) of biomass solids were lost during steam explosion. Cellulose remained almost unaffected (retained in fibers form) by water and alkali extraction. About 35-55 % of the hemicelluloses can be recovered in the water extracted liquor fraction (WEL). The remaining non-cellulosic carbohydrates were lost during steam explosion, especially at high severity. At Ro 10,000 and above, alkali extracted fibers (AEF) consists almost entirely of cellulose and non-carbohydrates. The majority of the non-carbohydrates component (> 50 %) can be isolated by alkali extraction. The non-carbohydrate component harvested increased with severity to 67% at Ro 35,000. / Master of Science