Dealloying is currently used to tailor the morphology and composition of nanoparticles and bulk solids for a variety of applications including catalysis, energy storage, sensing, actuation, super-capacitors and radiation damage resistant materials. The known morphologies, which evolve on dealloying of nanoparticles, include core-shell, hollow core-shell and porous nanoparticles. Here we present results examining the fixed voltage dealloying of AgAu alloy particles in the size range of 2-6 and 20-55 nm. High-angle annular dark-field scanning transmission electron microcopy, energy dispersive and electron energy loss spectroscopy are used to characterize the size, morphology and composition of the dealloyed nanoparticles. Our results demonstrate that above the potential corresponding to Ag+/Ag equilibrium only core-shell structures evolve in the 2-6 nm diameter particles. Dealloying of the 20-55 nm particles results and in the formation of porous structures analogous to the behavior observed for the corresponding bulk alloy. A statistical analysis that includes the composition and particle size distributions characterizing the larger particles demonstrates that the formation of porous nanoparticles occurs at a well-defined thermodynamic critical potential.