The transient structural response of laser excited gold nanoparticle sols has been recorded by pulsed X-ray scattering. Time resolved wide angle and small angle scattering (SAXS) record the changes in structure both of the nanoparticles and the water environment subsequent to femtosecond laser excitation. Within the first nanosecond after the excitation of the nanoparticles, the water phase shows a signature of compression, induced by a heat-induced evaporation of the water shell close to the heated nanoparticles. The particles themselves undergo a melting transition and are fragmented to Form new clusters in the nanometer range. (C) 2004 Elsevier B.V. All rights reserved.