Local variations in the cation composition along grain boundaries in bulk-scale YBa 2 Cu 3 O 7 - δ bicrystals grown by the flux method were investigated by high-spatial-resolution energy-dispersive X-ray spectroscopy (EDS) in a scanning transmission electron microscope (STEM). Composition profiles along 7° [001] and 31° [001] tilt boundaries reveal regularly-spaced regions that are rich in copper relative to both the grain interior and the neighboring sections of the grain boundary. The average spacing and range of spacings between adjacent copper maxima are 75 nm and 60-100 nm, respectively, for the seven maxima observed in the 7° bicrystal. Three maxima with a spacing of ∼ 100 nm were detected in a shorter segment of the 31° boundary, In both bicrystals, the spacing of the maxima equaled the wavelength of an approximately periodic strain field with dislocation array like character that was observed in the boundary. Long-wavelength strain fields of this type previously have been associated with saw-tooth-shaped facet structures in flux-grown YBa 2 Cu 3 O 7 - δ bicrystals. A saw-tooth facet structure was observed directly only in the 7° bicrystal, however. Less extensive investigations of additional bicrystals suggest that oscillatory cation nonstoichiometry is general to [001] tilt boundaries in flux-grown YBa 2 Cu 3 O 7 - δ bicrystals and possibly to more general boundaries. Regular variations in boundary nonstoichiometry are a possible origin for one scale of boundary subdivision and may contribute to the heterogeneity of the electromagnetic properties that is believed to occur on a range of different length scales in YBa 2 Cu 3 O 7 - δ .