In this paper, we study the protonation of pyridine and phosphinine derivatives. In particular, the geometries, the amount of charge transfer, and the nature of the created N-H and P-H bonds are discussed, underlying the fundamental differences between the phosphorus and the nitrogen atoms as proton acceptors. Conceptual density functional theory and Bader's quantum theory of atoms-in-molecules are notably used to rationalize these trends and to predict the overall energies of these prototype gas-phase acid-base reactions.