A thermodynamic derivation of the vapor pressure equation for curved surfaces (droplets) is presented here. It is shown that the vapor pressure depends not only on the usual variables (surface tension, droplet radius, temperature, and molar liquid volume) but also on the number of moles of the different phases. The present equation is valid for droplets having the same number of moles in the liquid and at the interfacial phase (limit of very small droplets). For droplets of standard size it leads to the Kelvin equation.