The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC.0.43Si(3)N(4).0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at pN(2) < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at similar to10 bar, and is crystalline above similar to10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si3N4 are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at similar to10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate alpha-Si3N4 crystals is calculated to be 30 kJ/mol.