We have recently extended the electron propagator theory to the treatment of any type of particle using an Any-Particle Molecular Orbital (APMO) wavefunction as reference state. This approach, called APMO/PT, has been implemented in the LOWDIN code to calculate correlated binding energies, for any type of particle in molecular systems. In this work, we present the application of the APMO/PT approach to study proton detachment processes. We employed this method to calculate proton binding energies and proton affinities for a set of inorganic and organic molecules. Our results reveal that the second-order proton propagator (APMO/PP2) quantitatively reproduces experimental trends with an average deviation of less than 0.41 eV. We also estimated proton affinities with an average deviation of 0.14 eV and the proton hydration free energy using APMO/PP2 with a resulting value of −270.2 kcal/mol, in agreement with other results reported in the literature. Results presented in this work suggest that the APMO/PP2 approach is a promising tool for studying proton acid/base properties.