Experimental data on grain growth and oxidation kinetics of SiC-based fibers, as well as the accompanying strength degradation, in argon, air, and moist air are interpreted using a mechanistic model. The grain growth from thermal history is modeled using conventional models, and its influence on strength is modeled assuming that the flaw size scales with grain size. The model for fiber oxidation uses available relevant thermodynamic and kinetic data for reactions, vapor pressures, oxygen permeation, and boundary layer effects to capture scale thickness data reported by several prior works, in static or flowing air, moist air, and steam. The effect of the oxide scale on strength was modeled assuming that the flaw size scaled with scale thickness. The resulting model is compared with experimental data and is shown to capture most of the data in the literature on degradation of HiNicalon and HiNicalon type S fibers.