Abstract The PCILO (Perturbative Configuration Interaction Using Localized Orbitals) method has been used to determine the electronic structure of the active center of serine proteases. The results show that the carboxyl group of the aspartic acid residue is the ultimate proton acceptor of the catalytic triad (Asp, His, Ser) −. In the absence of a substrate the negative charge of the active centre is delocalized, causing polarization of the Ser O γ-H bond and an increase of the nucleophilicity of the O γ atom. The proton of the O γ-H bond of the Ser residue is, however, only partially transferred to the N ϵ2 atom of imidazole His. The hydration of the model charge relay system is also investigated.