HfO2-SiCN (polymer-derived silicon carbonitride) composites were prepared by two methods. In one case, equal volume fractions of HfO2 and pyrolyzed powders of SiCN were co-sintered, to create a particulate composite. The second type, called interface composites, were prepared by coating HfO2 particles with a thin film of the polymer precursor, followed by sintering so that densification and pyrolysis of the precursor occurred simultaneously; this process results in a similar to 5-nm-thick grain boundary film constituted from Hf, O, and Si. The fracture properties and environmental degradation (in a humid environment at a velocity of 17.6-35.0 cm/s at 1300 degrees C) of these two composites were measured. They were compared with the properties of a reference material made by sintering HfO2 powders without any additives, under similar conditions (1450 degrees C for 2 h in air). The interface composite yielded the highest sintered density (0.90), exhibited negligible grain growth, and possessed the highest fracture strength (110 MPa). The strength remained immune to hydrothermal oxidation for several hundred hours. In contrast, the particulate composite suffered severe degradation in strength after hydrothermal exposure. The interface composites, with their highly refractory grain boundaries, represent a new class of ceramics for structural applications in harsh environments and at ultrahigh temperatures.