The coarsening processes which occur concurrently with the sintering of B4C were deduced using differential dilatometry, weight loss and grain size measurements. B2O3 coatings on B4C particles stalled the onset of densification until these coatings decomposed away at 1840degreesC. Oxide gaseous species formed in this process were postulated to be one conduit of particle coarsening. At higher temperatures, the volatilization of B4C itself were interpreted to be the second vapor phase coarsening mechanism, starting at temperatures above 2000degreesC. Carbon additions or soaking the compact in a hydrogen-containing atmosphere at 1350degreesC removed the oxide coatings at temperatures below those in which sintering or coarsening occurred. Accelerating densification above 2130degreesC was attributed to nonstoichiometric volatilization, leaving carbon behind to facilitate activated sintering. (C) 2004 Kluwer Academic Publishers.