We report on a method to measure the mean inner potential (V0) using transmission electron microscopy. It is based on phase retrieval from a focus series and has allowed to measure V0 as a function of the size for a system of gold nanoparticles. It comes out that V0 increases for particles below 2 nm. The focus series being carried out in conditions close to the high-resolution ones, structural information can be directly obtained. The high-resolution images have revealed that significant structural change occurs below the 2 nm size, which should be related to the V0 increase.