Hi-Nicalon-S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100hours between 700 degrees C and 1400 degrees C in wet and dry air. SiO2 scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO2 scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal-Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi-Nicalon-S fibers are compared with previous studies on SiC fibers, bulk SiC, and single-crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.