Recent focus on algae biomass as an alternative energy source can be attributed to building pressure for conservation of dwindling fossil fuels and reduced greenhouse gas emissions. Both micro- and macroalgae have many advantages over terrestrial plants, including-typically faster growth rates and, therefore, higher rates of carbon fixation. This paper reports the combustion characteristics of a species of microalgae and two species of macroalgae under conditions that are relevant for the large-scale use of biomass for heat and other products. The tested species were Tetraselmis sp. (marine microalgae), Derbersia tenusissima (marine macroalgae), and Oedogonium sp. (freshwater macroalgae). Two variants of Oedogonium were tested. One variant was cultivated using standard additions, and the other variant was starved of essential nutrients. Carbon conversion to CO and CO2 and the release of N as NO were determined for the algae by oxidizing-fixed-bed samples of each alga in air at 800 and 1000 degrees C. The gasification reactivity of the chars was also characterized by gasifying samples of each alga in a thermobalance in pure CO2 (1 atm) at 800 degrees C, following in situ devolatilization of the algal samples. Carbon conversion to CO and CO2 exceeded 84% for all of the tested algae. Most of the fuel C was released during fuel devolatilization, consistent with the proximate analysis for these fuels. Nitrogen conversions to NO ranged between 6 and 12 g of N/100 g of fuel N for Tetraselmis, 6-9 g of N/100 g of fuel N for Derbersia, and 11-21 g of N/100 g of fuel N for the two Oedogonium variants, with NO emissions occurring mainly during devolatilization, in most cases. Chars produced from samples of macroalgae were much More reactive than the chars from the microalgae, Most likely because of the inhibitory effects on mass transfer caused by the very high ash content of the sample used in the present study. The reactivities of all chars increased at high char conversions.