We report results from a systematic study of the gasification of the alga Nannochloropsis sp. in supercritical water from 450 °C to 550 °C. The gaseous products were mainly H 2, CO 2 and CH 4, with lesser amounts of CO, C 2H 4, and C 2H 6. Higher temperatures, longer reaction times, higher water densities, and lower algae loadings provided higher gas yields. The algae loading (wt%) 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 the intermediate products that were identified and quantified indicated that some products (e.g., alkanes) reacted quickly whereas others (aromatics) were converted 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. We show that a kinetics model built upon this reaction network is able to provide a quantitative accounting for all of the experimental trends observed.