Dried, milled corn fiber (0.2-0.5 kg) was fractionated by treatment with either hot liquid water (3-4 kg) at low solids loadings (5-10%) or steam (0.1-0.4 kg) at high solids loadings (> 50%) at 210-220 degreesC for 2 min, using the same novel process equipment. Pentosan recovery and inhibition of yeast fermentation were evaluated and compared. In addition, the reactivity of pretreated fiber with respect to enzymatic hydrolysis was evaluated using a simultaneous saccharification and fermentation (SSF) system consisting of P-glucosidase-supplemented Trichoderma reesei cellulase together with fermentation by Saccharomyces cerevisiae. Greater solubilization was achieved at 215 degreesC with hot liquid water at 5% solids loading than with steam at 70% solids loading (54% solubilization vs 37%). The lignocellulosic residue from this hot liquid water fractionation was enriched in glucan. Conversely, the steam fractionation caused no significant change in the fraction of glucan in the residue, relative to the feed material. In both cases, the pentosan fraction of the lignocellulosic residue was reduced, with the steam fractionation resulting in a larger reduction of these carbohydrates. Steam fractionation (70% solids loading) resulted in much lower pentosan recovery (as monosaccharides after posthydrolysis) than fractionation with hot liquid water at 5% solids loading (40% vs 82%). In both cases, the majority of the solubilized pentosans existed as oligomers (> 80%). These recoveries indicate that the monosaccharides are protected while in the form of oligomers because recoveries of largely oligomeric carbohydrates are higher than recoveries of monomeric xylose pretreated under similar conditions. The lignocellulosic residues from fractionation at 215 degreesC with hot liquid water at 5% solids loading and with steam at 70% solids loading were both reactive to enzymatic hydrolysis, exhibiting 86 and 90% conversion of glucan to ethanol respectively vs 64% conversion with untreated corn fiber, obtained by SSF at an enzymatic loading of 15 FPU cellulase/g of cellulose. The liquid product (extract) from the hot liquid water fractionation did not inhibit the final yield of glucose fermentation by S. cerivisiae, while the liquid product from the steam fractionation did.