In YBa2Cu3O x , muon spin rotation (μ+SR) measurements were taken at applied magnetic fields along the c-axis of 0.05, 1.0, 3.0, and 6.0 Tesla. These measurements, which take into account the expected field-dependent and temperature-activated flux-line disorder, are consistent with a nodeless (s-wave) superconducting order parameter, and inconsistent with order parameters possessing nodes, including those with $$d_{{x}^2-{y}^2}$$ symmetry. We conclude that the superconductivity is p-type and occupies the BaO layers of YBa2Cu3O x . In other cuprates orruthenates, thesuperconductivity resides in the BaO, SrO, or interstitial oxygen regions.