Silica thin films (similar to1 mum) have been obtained with embedded Ag nanoparticles with a controlled size distribution, using U chemical approach. Our method is able to give Ag concentration in the range of 0-15% in weight. Several characterization techniques reveal that the particle average size is independent of silver concentration. In this paper, we report a detailed analysis of the nonlinear optical properties of these nanocomposite thin films as a function of the Ag cluster density. These analyses are made in the nanosecond regime at 532 nm. We find two different nonlinear regimes by changing the silver concentration. At low particle density we observe an optical limiting effect, while at higher density values the nonlinear absorption coefficient changes its sign due to a saturable absorption (SA) process. A tentative explanation of the observed phenomenology is given in terms of particle-particle interaction. (C) 2003 Elsevier B.V. All rights reserved.