AbstractFor many proteins, phosphorylation regulates their interaction with other biomolecules. Herein, we describe an unexpected phenomenon whereby phosphate groups are transferred non‐enzymatically from one interaction partner to the other within a binding interface upon activation in the gas phase. Providing that a high affinity exists between the donor and acceptor sites, this phosphate transfer is very efficient and the phosphate groups only ligate to sites in proximity to the binding region. Consequently, such phosphate‐transfer reactions may define with high precision the binding site between a phosphoprotein and its binding partner, as well as reveal that the binding site in this system is retained in the phase transfer from solution to the gas phase.