Black liquor droplets, initially 1-3 nun in diameter, were burned while simultaneously measuring surface and internal temperature, mass loss, and diameter to gain insight into the physical processes of single droplet combustion. Data were collected by weighing a droplet hanging on a thermocouple wire within a heated furnace. Two-color images of the droplet were obtained to determine a surface temperature and particle size. The data suggest that models of droplet combustion should treat drying, devolatilization, and char oxidation as occurring simultaneously, though typically in different regions of the particle. Isothermal models should be avoided as temperature differences in excess of 300 K exist between the surface and interior, with larger gradients along the surface. Droplet swelling increased at lower ambient temperatures, and is greater for softwood than hardwood liquors. Additional unidentified factors also influence swelling, meaning it is still highly unpredictable and may be a function of initial droplet shape, size, and chemical composition. (C) 2008 American Institute of Chemical Engineers.