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Interface states and lateral nonuniformities produce very similar abnormalities in the curves of MIS capacitors. The effect of a laterally nonuniform distribution of fixed charge in the insulator can be characterized by a flat-band-voltage distribution function. Here we present a simple, approximate method for determining this distribution from the quasi-static and high-frequency curves of the capacitor, and we apply the result to a test for distinguishing between interface states and laterally nonuniform fixed charge. The test is based on a principle that is implicit in the results of an analysis previously published by Brews and Lopez; namely, that interface states and lateral nonuniformities cannot produce identical distortions in both the quasi-static and high-frequency curves. The presence of either lateral nonuniformities or interface states can be tested by assuming all distortion to be due to the other cause. The curves are regenerated under this assumption, and discrepancies between the measured and regenerated curves indicate the presence of the effect assumed not to be present.