We have investigated interlayer coupling of Co/Ge superlattices. The present experiments obviously show that the coupling changes from ferromagnetic (F) to antiferromagnetic (AF) and finally to non-coupling (N) with the increase of Ge layer thickness. This coupling behaviour, as a function of the spacer thickness, is very similar to that of Fe/Si superlattices, although the coupling strength is much smaller than the latter: namely, approximately 0.05 erg cm-2 for Co/Ge and approximately 1.0 erg cm-2 for Fe/Si. Precise structural characterization indicates that diffused spacers at Co/Ge interfaces are responsible for the AF coupling. The same coupling behaviour has also been observed in Co/non-magnetic Co-Ge superlattices, where interdiffusion at the interfaces is entirely suppressed. All these results clearly demonstrate that the interlayer coupling between neighbouring Co layers is mediated by non-magnetic Co-Ge spacers.