Molecular-beam-epitaxy-grown hexagonal Fe/Ru superlattices have been investigated by anomalous x-ray diffraction near the iron absorption K edge to inquire selectively into the structure of both the iron and the ruthenium sublattices. The data have been analyzed by comparing simulated and experimental spectra in k space at various wavelengths and by means of a difference Fourier-transform method that allows us to trace the iron density profile along the growth direction of the superlattice and then directly probe the Fe/Ru interface. From both methods we extract the interplanar spacing for Fe and Ru. We get c(Fe) approximately 4.11 angstrom and C(Ru) approximately 4.33 angstrom. The value a approximately 2.69 angstrom is the same for both sublattices. We can thus calculate the atomic volume of hexagonal Fe in the superlattice V(Fe) approximately 12.87 angstrom3 which is expanded by 14% with respect to hcp Fe. This may explain the onset of ferromagnetism in this metastable hexagonal Fe phase.