Characterization of by-products generated during organosolv pretreatments of yellow poplar wood was performed in the presence of sulfuric acid and sodium hydroxide catalysts to elucidate the effects on further biological fermentation and provide the preliminary data for utilization of organosolv lignin. Monomeric sugars contents were 2.2.7.7% in organosoluble fraction with acid catalyst, while significantly lower amount of sugars (0.2.0.3%) were determined in that of with alkali catalyst. However, the concentrations of monomeric sugars were very low to ferment directly. Therefore, the oligomeric sugars in organosoluble fraction should be recovered to improve the economic feasibility of the organosolv pretreatment. Higher concentrations of inhibitory compounds such as acetic acid, 5-HMF and furfural were determined in the organosoluble fraction with acid catalyst. However, the concentrations were lower than the minimum inhibitory concentration to the microorganism for further biological process regardless of catalyst. Thus, the compounds could not inhibit the biological fermentation of organosoluble fraction. Precipitates, solid by-products, were also characterized by diverse analytical methods and the analysis results revealed that the precipitates weremostly composed of lignin polymers regardless of catalyst. However, the structures could be slightly modified during pretreatment and mixed with come carbohydrates by chemical bonds and/or physical association.