The composition, crystallinity, uniformity, purity and thermal stability of cuprate superconductors have been studied by Rutherford backscattering and channeling spectrometry, and 3.045 MeV He + oxygen non-Rutherford resonant scattering. Further experiments have been performed with 1.75 MeV H + carbon non-Rutherford resonant scattering. Three sets of samples were studied: HgBa 2CuO (4+ δ) (Hg1201), Hg x Tl 1− x Ba 2Ca 2Cu 3O (2 n+ δ) (Hg,Tl-1223) and Tl 1.85Ba 2CuO 6/LaAlO 3 (Tl-2201), either in bulk or as an epitaxial thin film. It was observed that the superconductors exhibit a metal deficiency near the surface, which is largely compensated by excess oxygen. Moreover, the samples are significantly contaminated with carbon within the probing region of the H + beam. The thermal stability and surface degradation were studied in both oxidizing ambient and vacuum. As a general trend, the heavy metal deficiency – and consequently the compensating oxygen excess – is enhanced as the temperature increases.