A zone-axis critical voltage is reported in the [001] transmission electron diffraction pattern of the superconducting ceramic La1.8Sr0.2CuOy. Experimentally, this is found to occur at 95 kV; simplified theoretical calculations indicate that the critical voltage is so highly sensitive to the crystal potential that it changes by > 5 kV for a 2.5% variation in oxygen vacancy content. In principle, this technique can now be used to measure the oxygen stoichiometry to within ±2.5% from a ∼ 1 nm region in a well characterized single-phase region, and should also provide a sensitive measure of the La stoichiometry. Problems with the absolute quantification of the method are discussed, and it is concluded that the major application of this effect lies in comparative measurements of stoichiometry variations from sample to sample, and compositional inhomogeneities within a single sample.