Small, mass-selected Cr-n clusters soft landed onto a (1 x 1)O/Ru(001) substrate were investigated by near-edge X-ray absorption spectroscopy. Cr-n X-ray absorption spectra exhibit size-dependent shifts of L-3 absorption edges. Furthermore, strong cluster-substrate interaction of Cr-n clusters deposited onto (1 x 1)O/Ru(001) results in Cr2O3-like absorption signatures and effectively inhibits diffusion and island formation even at elevated temperatures. In contrast, chromium atoms on Ru(001) have a high mobility, showing indications of coalescence and island formation; the resulting larger clusters can easily be oxidized.