We report the synthesis, structural characterization, and optical studies of ultrathin Ge-Si superlattices with individual sublayers smaller than the Si unit cell, grown by MBE on (001) silicon substrates. Structures are fabricated one monolayer at a time in a configuration GeGeSiSiGeGe. . . , resulting in either ordered alloys or complex cell superlattices. Rutherford backscattering and channeling experiments on these heterostructures indicate excellent crystallinity with tetragonal distortion as high as 3. 5%. Electron diffraction patterns exhibit characteristic superlattice reflections indicative of one-dimensional layering with periodicity of four monolayers. X-ray scans along the growth direction at the (002) position in reciprocal space reveal a strong peak not observed in random GeSi alloys. This scattering is attributed indirectly to the GeSi ordered phase.