Ferromagnetic resonance (FMR) is used to probe the magnetic properties of Co / Ag superlattices (SLs) with ultrathin Co layers (2–6 Å ) . Different series of 5×[ Ag / Co ] multilayers have been grown by molecular beam epitaxy on Si(111) substrates, monitoring the growth by reflection high energy electron diffraction. Cross-section transmission electron microscopy confirms the growth of local areas with the designed SL periodicity, a sharp compositional modulation, well defined Ag–Co interfaces, and a perfect fcc (111) stacking. FMR spectra have been recorded at various polar angles in the 0°–90° range. A single and extremely broad resonance peak is observed in all cases. While SLs with Ag layers thinner than 10 Å exhibit similar values of the perpendicular anisotropy, a clear reduction is observed for samples with Ag layers about 14 Å thick. Possible causes for this change are discussed.