The nonlinear optical properties of Si nanoclusters formed by ion implantation into an SiO2 glassy matrix and followed by annealing have been studied at λ=532 nm and λ=355 nm by use of Z-scan and pump-probe techniques. These have been compared to the nonlinear properties of laser-ablated Si films. At relatively large intensities (≫1 MW/cm2), the absolute nonlinear values for these isolated nanoclusters were comparable to those obtained for laser-ablated samples although opposite in sign. Laser-ablated samples showed a much larger effect at relatively low intensities (≪1 MW/cm2), while the ion-implanted films showed almost none. Lifetime constants were in the range of 3–5 ns for all samples. © 1998 American Institute of Physics.