We determined the effects of different process variables on the gasification of Nannochloropsis sp., a marine microalga, in water with a Ru/C catalyst at 410 degrees C. About 45% gasification efficiency was achieved at 75 min with a catalyst loading (mass of Ru/C catalyst/mass of dry algal biomass) of 1 g/g, a water density of 0.096 g/cm(3), and a 4.3 wt % loading of algae. Longer reaction times, higher catalyst loadings and water densities, and lower algae loadings provided higher gas yields. The level of catalyst loading had the most significant effect on both the yields and composition of the gaseous products. Complete gasification of the microalga was achieved with a catalyst loading of 2 g/g. The Ru/C catalyst recovered from an experiment and then reused in a new experiment was less active than fresh Ru/C. This loss of activity is, in large part, due to deactivation by sulfur, which is present in the microalga at about 0.5 wt %. A simple two-step catalytic gasification mechanism, along with a step for catalyst poisoning by sulfur, led to a rate equation that was consistent with the experimental results for gasification efficiency. The presence in algae of sulfur and perhaps other elements, such as Cl, that are not as prevalent in terrestrial biomass indicates that efficient and effective gasification of microalgae could present new challenges in engineering and catalyst design.