The swelling behaviour of pine and birch kraft black liquors and their main organic components (lignin and hydroxy carboxylic acids) was studied during heating treatment (25-->600 degrees C; 30-40 degrees C min(-1)) under a stagnant inert atmosphere. The black liquors began to swell considerably in the temperature range of 180-270 degrees C. No marked differences in the swelling properties of the two liquors were found. On the basis of a comparison of the swelling behaviour of the organic components of the black liquors to that of the whole liquors, it was concluded that the aliphatic acid fraction is the main source of the volatile degradation products necessary for swelling, while the lignin fraction has an essential role in hindering the escape of the volatile compounds from within the particle. The study was carried out in support of the development of a process for producing gaseous fuels from spent pulping liquors.