Hydrothermal carbonization (HTC) can potentially improve the fuel quality of low-value biomass resources that are otherwise unsuitable for use in industrial heat and power applications. The effect of HTC pretreatment on the combustion and gasification behavior of two agricultural residues (grape marc and sugar cane bagasse) and a freshwater species of macroalgae was investigated, with an emphasis on devolatilization behavior and char reactivity. HTC was carried out in a custom-built, laboratory-scale, batch reactor at three temperatures, 180, 220, and 260 degrees C, with a slurry density of 15\% w/w dry biomass in water. The volatile release behavior of the collected products (hydrochars) and untreated biomass feedstocks were characterized by dynamic thermogravimetric analysis. Char reactivity was characterized by isothermal gasification of samples of the hydrochars and untreated biomass in a thermobalance in carbon dioxide, following fast pyrolysis of the samples in a separate fixed-bed batch reactor. Hydrochars were more energy-dense and contained lower concentrations of catalytic metals, particularly K and Na, than the untreated biomass feedstocks. HTC caused a significant reduction in the total release of volatiles and an increase in the yield of char during devolatilization. The bulk of the volatile matter was released at higher devolatilization temperatures for the hydrochars. These trends became more pronounced with increasing HTC temperature from 180 to 260 degrees C. The char components of grape marc and macroalgae became substantially less reactive following HTC. The char component of bagasse became more reactive following HTC at 180 degrees C but became less reactive following HTC at 220 and 260 degrees C. Activation energies for char gasification of HTC-treated (220 degrees C) grape marc, bagasse, and macroalgae in CO2, were 177, 247, and 282 kJ mol(-1), respectively. These values are 92\%, 10\%, and 52\% greater than the values for the untreated biomass.