The reactions of a strain of marine alga Chlorococcum sp. and of a polystrain containing a mixture of marine algae with CO/H2O have been studied. The yields and chemical composition of the products obtained from the reactions with CO/H2O were compared with those using N-2 and H-2. The effects of selected potential catalysts (Na2CO3, NaAlO2, and Fe) and the water-to-algae ratio on the product yields and composition when the reactions were carried out under CO/H2O are also described. Both algae gave high product yields at 285 degrees C, but the quality of the product as a liquid fuel increased at higher temperature. The reactions with CO/H2O tended to give higher oil yields and higher-quality products (lower nitrogen, higher atomic H/C ratio) than those using H-2 or N-2 and, in addition, reduced the amount of dewatering of the algae that is required.