Fluid phase equilibria involving nano dispersed phases, where at least one of the coexisting phases is confined to a small volume, are investigated by molecular dynamics simulation. Complementing previous studies on nanoscopic droplets, simulation volumes containing a nanoscopic gas bubble surrounded by a subsaturated liquid phase under tension, i.e. at negative pressure, are conducted in the canonical ensemble. The boundary conditions are chosen such that the phase equilibrium at the curved interface is thermodynamically stable. Two distinct size dependent effects of opposite sign are found for the density of the gas in the centre of the bubble. The curvature dependence of the surface tension is considered, employing an approach based directly on the average radial density profiles. (C) 2014 Elsevier Ltd. All rights reserved