Hydrolysates produced by the pretreatment of aspen wood with liquid hot water were compared with hydrolysates produced using carbonic acid pretreatment. Pretreatment temperatures tested ranged from 180 to 220 C; reaction times were varied between 2.5 and 30.5 min. Under most conditions tested, it was found that the presence of carbonic acid had no discernible effect on the amount of xylose released or concentration of furan compounds, as indicated by ultraviolet-visual absorbance between 270 and 280 nm. Thus, there appears to be little difference in the severity of the pretreatment conditions with or without the presence of carbonic acid. The presence of carbonic acid did, however, result in a hydrolysate with a higher final pH. It is hypothesized that the presence of the carbonic acid during the reaction may have the effect of reducing the accumulation of organic acids in the hydrolysate.