The reduction of aromatic and aliphatic (di)carboxylic acids to their corresponding aldehydes and alcohols by the hyperthermophilic organism Pyrococcus furiosus was investigated. The reduction was performed with P. furiosus cells growing in the presence of 1 mM acid with starch as a carbon and energy source at 90 degrees C. The aromatic acids t-cinnamic and 3-phenylpropionic acid were reduced to their corresponding alcohols with the highest yields in the described batch cultures: 67 and 69%, respectively. The aliphatic acid reduced with the highest yield was hexanoic acid (yield: 38%). No aldehydes were detected-during the reduction of acids, indicating that the reduction of aldehydes to alcohols is faster than the reduction of acids to aldehydes. Some aldehydes were both reduced to the corresponding alcohol and oxidized to the corresponding acid. Besides reduction of the unsaturated t-cinnamaldehyde to t-cinnamyl alcohol (63%), the double bond of t-cinnamaldehyde was also reduced by P. furiosus.