The hitherto unknown gas-phase basicity of the tetraphosphorus molecule, P-4, has been determined by means of Fourier transform ion cyclotron resonance mass spectrometry using proton transfer equilibrium techniques. A quantum mechanical treatment of P-4 and P4H+ in the framework of the G2 level of theory leads to a proton affinity agreeing nicely with the experimental value and, most important, reveals the existence of a novel kind of chemical bond in P4H+ : the symmetrical and essentially covalent (P ... H ... P) linkage.