A unified quantum-mechanical model of contact electrification that provides a microscopic basis for the Volta–Helmholtz–Montgomery hypothesis is presented. The model can represent metals, semiconductors, or insulators, in either fluid or solid phase, and with an effective electron transfer parameter as the driving mechanism. Known experimental results such as the charging of similar materials, the charging of similar materials with different contact orientation, the surface charge mosaic, and the higher efficiency of charge transfer for a liquid–solid contact, compared to a solid–solid one, are reproduced. A quantum-mechanical charge oscillation in the femtosecond to picosecond regime is predicted to take place. Coulomb interaction is found to have an impact on not just the charge transferred but also the period of charge oscillation.