The design and synthesis of Al-Cr-O-N thin films in a metastable solid solution strengthened α-(Al1-x,Crx)2+δ(O1-y,Ny)3 microstructure are described. The deposition experiments were carried out by reactive r.f. magnetron sputtering in an argon-oxygen-nitrogen atm. for the sputtering from a segmented target, composed of an Al half plate and a Cr half plate. The constitution, microstructure and Vickers micro hardness of the coatings are discussed in dependence of the deposition parameters and the elemental composition The range of α-(Al1-x,Crx)2+δ(O1-y,Ny)3 phase formation is shifted to higher Cr/(Al + Cr) ratios with increasing nitrogen gas flow. A γ-Al-Cr-O-N phase is formed at high total gas pressures and low Cr/(Al + Cr) ratios. The solid solution strengthening in the quaternary material system leads to a significant Vickers micro hardness increase compared to α-(Al1-x,Crx)2O3 coatings deposited in the same PVD machine under corresponding process parameters. The α-(Al1-x,Crx)2+δ(O1-y,Ny)3 thin films offer an addnl. restraint of the lattice by the incorporation of nitrogen and therefore higher Vickers micro hardness values are measured compared to the Al-Cr-O material system.