The primary aim of this research work was to characterize the swelling properties of black liquor and their dependence on the temperature and gas composition of the combustion or gasification environment. The measurements were made using an industrial black liquor from soda pulping of wheat straw. The swelling measurements were carried out in a single droplet reactor. This paper presents data that show the influence of the reactor temperature and the gas composition on the swelling during devolatilization, and compares the swelling characteristics of a straw black liquor with similar published data on black liquors from kraft pulping of wood, Experiments were carried out in inert, oxiding, and gasifying environments, at temperatures from 500 to 900 degreesC. One interesting result was that the straw liquor swelled much more than most kraft or soda liquors from pulping of wood. Another result was that droplets subjected to hot gas atmospheres containing O-2, CO2 and/or H2O(V) swelled less than those subjected to hot N-2 atmospheres. The data obtained suggest that the straw soda black liquor would burn more rapidly than most kraft liquors from pulping of wood because of its high external surface area when swollen. This highly swelling liquor caused problems with fluidization when straw liquors were injected in a laboratory-scale fluidized bed gasifier.