Nanocrystalline Ti1-x AlxN (0 less than or equal to x less than or equal to 0.41) solid solution films were produced by reactive unbalanced close-field magnetron sputtering. Nanoindentation measurements showed that the hardness of Ti1-x,AlxN films increased monotonously with the content of Al. A calculation based on a semiempirical method revealed that the effect of intrinsic hardening, which arises from the change of the nature of atomic bonding due to the incorporation of Al atoms into TiN lattice, played a negligible role in the observed hardening phenomena. Further analysis revealed that the grain boundary hardening was also very weak and the improvement of hardness of Ti1-xAlxN films with relatively low content of Al (x less than or equal to 0.33) could be well explained by the Fleischer model of solid solution hardening. However, for Ti1-xAlxN films with x>0.33, an obvious deviation from the solid solution hardening was observed, probably due to the grain boundary segregation of solutes that might lead to an enhanced effect of grain boundary hardening when the amount of Al is high. (C) 2004 Elsevier B.V All rights reserved.