The structural characteristics of Bi1.84Ca2.00Co1.88Ox and Bi2.49Sr2.00Co2.22Ox are studied by electron diffraction measurements and high-resolution electron microscopy. The crystal structures consist of two interpenetrating subsystems of a CdI2-type CoO2 sheet and a distorted four-layered rock-salt (RS)-type block. Both subsystems have common lattice constants a and c and a angle. The crystal structure is incommensurately modulated parallel to the b-axis. These compounds have modulation vectors, q1 = -a* + 0.58[image omitted] and q2 = 0.18[image omitted] + 0.5c* for the Ca system, and q1 = -a* + 0.54[image omitted] and q2 = 0.05[image omitted] for the Sr system. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated atomic arrangements. The local modulation mode is related to the shear deformation in the a-b plane.
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