We show that in the mixed-valence 3d transition-metal oxide NaV2O5 undergoing a structural transition, the low-temperature phase results from an effective ordering of the charge. This arrangement and the quantitative evaluation of the atomic charges are determined by using resonant x-ray scattering experiments further analyzed with the help of realistic calculations of the corresponding scattering factors. We have found that this reorganization concerns only a small fraction of electron and is necessary to reconcile all the experimental data.