Energy transfer from photoexcited nanoparticles to their surroundings was studied for both hollow and solid gold nanospheres (HGNs and SGNs, respectively) using femtosecond time resolved transient, extinction spectroscopy. HGNs having outer diameters ranging from 17 to 78 nm and fluid filled cavities were synthesized by a sacrificial galvanic replacement method. The HGNs exhibited energy transfer half times that ranged from 105 +/- 10 ps to 1010 +/- 80 ps as the total particle surface area increased from 1005 to 28 115 nm(2). These data showed behaviors that were categorized into two classes: energy transfer from HGNs to interior fluids that were confined to cavities with radii <15 nm and >= 15 nm. Energy transfer times were also determined for solid gold nanospheres (SGNs) having radii spanning 97-30 nm, with a similar size dependence where the relaxation times increased from 140 +/- to 310 +/- 15 ps with increasing nanoparticle size Analysis of the size-dependent energy transfer half dines revealed that the distinct relaxation rate constants observed for particle-to-surroundings, energy, transfer for HGNs with small cavities were the result of reduced thermal, conductivity of confined fluids. These data indicate that the thermal conductivity of HGN cavity-confined fluids is approximately one-half as great as it is for bulk liquid water. For all HGNS and SGNs studied, energy dissipation through the solvent and transfer across the particle/surroundings interface both contributed to the energy relaxation, process. The current data illustrated the potential of fluid-filled hollow nanostructures to gain insight into the properties of confined fluids