Xylonic acid was produced efficiently from pure xylose by Pseudomonas fragi ATCC 4973 and Gluconobacter oxydans subsp. suboxydans ATCC 621. The yield from 10% xylose was in both cases over 95% of the theoretical. However, the sensitivities of the strains towards the major inhibitors found in hemicellulose hydrolyzates, ie. acetic acid, furfural and two lignin-derived compounds, varied. G. oxydans tolerated all these inhibitors better than P. fragi. In tests using steamed hemicellulose hydrolyzate, G. oxydans was able to utilize the substrate only at dilute xylose concentrations. After ether extraction or mixed bed resin pretreatment, the fermentability of the hydrolyzate was increased significantly.