Single-phase [NaCl]-structure Ti(1-x)Zr(x)N thin films (0 < x < 1) have been deposited using cathodic arc plasma deposition. The films were investigated using X-ray diffraction (XRD), transmission electron microscopy, differential scanning calorimetry (DSC), and nanoindentation. Density functional theory calculations on phase stabilities show that the pseudo-binary TiN-ZrN system exhibits a miscibility gap, extending over 0 <= x <= 0.99 at 1000 degrees C, with respect to phase transformation from a solid solution into a two-phase mixture of [NaCl]-structure TiN and ZrN components. The films were found to retain their as-deposited single-phase structure during post-deposition annealing at 600 degrees C (18 h), 700 degrees C (12 h), 1100 and 1200 degrees C (2 h), and for as long as 195 h at 600 'C. DSC revealed no heat flow during annealing, similar to TiN, and only the x=0.53 film exhibited a slight increase in XRD peak broadening after annealing at 1200 'C, consistent with spinodal decomposition. This effective thermal stability of the alloys is explained by the combination of a limited driving force for phase transformation and an insufficient atom diffusivity. In terms of mechanical properties, films with composition deepest within the miscibility gap showed a hardness of similar to 30 GPa after annealing at 1100- 1200 degrees C; a value only moderately lower than in the as-deposited condition. The principal hardening mechanism for the Ti(1-x)Zr(x)N films is proposed to be solid-solution hardening through local lattice strain fields originating from difference in atomic radius of Ti and Zr. The material system is thus promising for cutting tool applications. (c) 2008 Elsevier B.V. All rights reserved.