Deposition of 0.5 monolayer of Mn by molecular beam epitaxy on the surface of a Ge(001) substrate, and annealing, allowed the fabrication of a cluster-free Ge(Mn) diluted solution. Electronic spin resonance (ESR) was used to study the magnetic properties of this solution. These measurements, combined with secondary ion mass spectrometry, atomic force microscopy, and Auger electron spectroscopy, show that the detected ferromagnetic signal is due to surface islands, while Mn atoms on Ge substitutional sites gives no detectable ESR signal. •A diluted Ge(Mn) solid solution was prepared without Mn–Si clusters.•The magnetic property of this solution was studied by electronic spin resonance.•A ferromagnetic signal was measured due to Mn–Ge islands on the sample surface.•Mn atoms located on the Ge crystal substitutional sites are not ferromagnetic.