The temperature dependence of the viscosity and stress relaxation kinetics of sol- gel-derived SiOC glasses that contain up to 14 at.% carbon have been characterized in the temperature range of 1000 degrees -1400 degreesC, The viscosity, as determined from relaxation experiments, is in good agreement with the creep viscosity and is typically two orders of magnitude higher than the viscosity of vitreous silica, However, materials suffer from partial crystallization at > 1150 degreesC, and the precipitation of beta -SiC nanocrystals induces a flow-hardening behavior and results in a dynamic increase in viscosity, especially at > 1200 degreesC and for glasses with a high carbon content.