We report results from a systematic study of the gasification of the alga Nannochloropsis sp. in supercritical water at 450-550 degrees C. The gaseous products were mainly H-2, CO2, and CH4, with lesser amounts of CO, C2H4, and C2H6. Higher temperatures, longer reaction times, higher water densities, and lower algae loadings provided higher gas yields. The algae loading strongly affected the H-2 yield, which more than tripled when the loading was reduced from 15 wt% to 1 wt%. The water density had little effect on the gas composition. The temporal variation of intermediate products indicated that some (e.g., alkanes) reacted quickly whereas others (aromatics) reacted more slowly. On the basis of this observation and the complete set of experimental results, we propose a global reaction network for algae SCWG that includes parallel primary pathways to each of these two types of intermediate products. The intermediate products then produce gases. (C) 2011 Elsevier B.V. All rights reserved.