Sputter-deposited thin films of Ti-B-N with different B contents were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nanoindentation measurements in order to investigate the microstructure transformation process upon increasing B content and to find the optimal microstructure corresponding to the highest hardness. The results indicated that part or all of the four phases, TiB, TiB, TiN and BN, existing in the reactively sputtered Ti-B-N thin films, depending on the B content. The films with a small amount of B addition were essentially cubic B1-NaCl TiN with (111) preferred orientation to the substrate surface, plus small amounts of TiB and BN phases. As the B content increased, the TiB phase gradually transformed to the TiB phase. After TiB vanished, the BN phase increased sharply at the expense of the TiN phase. The maximum hardness was achieved by the formation of a multiphase microstructure comprising of a mixture of three phases with the following mole fractions ∼73 mol.% TiN, 17 mol.% TiB, 10 mol.% BN. The effect of B on stabilizing the multiphase microstructure of TiN, TiB and BN was elucidated and explained on the basis of structural and thermodynamic stability. © 2004 Elsevier B.V. All rights reserved.