The fast component of the nonlinear refractive index, n(2), of a composite film formed by alloy nanoparticles embedded in silica was measured by Z-scan technique, in conditions free from cumulative heating effects. By using a single 6 ps long laser pulse at 1 Hz, 527 nm of wavelength, a negative sign of n(2) was evidenced. An explanation of this result is suggested, based on the dielectric confinement effect. The fast refractive index value, in modulus, is of the order of 10(-10) cm(2)/W. (c) 2005 Elsevier B.V. All rights reserved.