Characterization of superconducting Bi–Sr–Ca–Cu oxides has been carried out by electrical measurements, x-ray diffraction, conventional and high-resolution electron microscopy, and electron microprobe analysis. Nominal starting compositions with cation ratios of 1:1:1:2 and 2:2:1:2 show considerably different superconducting behavior. In both cases multiphase materials are formed. The predominant superconducting phase occurs as thin platelets with an orthorhombic, modulated structure. These particles often have edges aligned along , , and  directions and contain subgrain boundaries. Electron diffraction patterns and high-resolution micrographs taken along several zone axes are consistent with an incommensurate centered modulation along the b axis with a magnitude of 4.7 ± 0.1 times b. Unexpectedly, two distinct chemical compositions were found in platelets with the same apparent structure: Bi4Sr3Ca3Cu4O16±δ for the lower Tc phase in the 1:1:1:2 material, and Bi2Sr2CaCu2O8±δ for the isomorphic higher Tc phase present in the 2:2:1:2 samples.