Fermentation techniques for conversion of dilute acid hydrolyzates were examined. Batch and fed-batch fermentations of hydrolyzates from spruce and birch woods were made in a lab-scale (3.3 l) anaerobic bioreactor using the yeast Saccharomyces cerevisiae. The spruce and birch hydrolyzates contained high initial concentrations of furfural (2.2 and 5.7 g/l) and 5-hydroxymethylfurfural (HMF, 7.3 and 2.4 g/l), and were found to be strongly inhibiting to the yeast strain used in this study. Fermentation of the hydrolyzates was not possible using a batch mode of operation. However, using a fed-batch technique with a suitably adjusted feed rate, it was found possible to completely ferment the glucose and mannose sugars in both hydrolyzates. Most of the furfural (90%), and part of the HMF (40-70%), present in the hydrolyzates was converted during the fed-batch operation. It is suggested that the success of the fed-batch operation is related to the conversion of furfural and HMF.