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The monotonic and high-frequency (100 Hz) fatigue behavior of two Nicalon-fabric-reinforced SiCON matrix composites was investigated at room temperature. The matrix composition was varied by the addition of BN and SiC particulate fillers, to contain shrinkage from processing by polymer infiltration and pyrolysis (PIP). The composites had strong fiber/matrix bonding, which resulted in substantially less frictional heating than observed with weakly bonded composites. Both composites exhibited fatigue runout at 10 7 cycles at ∼80% of the monotonic strength. Comparison with existing fatigue data in the literature (for the same composites) at 1 Hz shows no change in fatigue life; i.e., no frequency effect was observed. Most of the stiffness reduction in the composites occurred in the first fatigue cycle, whereas subsequent decreases in moduli were attributed to limited fiber cracking. The major driving force for failure was the localized debonding of transverse and longitudinal plies at the crossover points in the fabric, which, when linked, resulted in interlaminar damage and failure in the composite.