The acidity and basicity trends in the series of alpha,beta-unsaturated amines, phosphines, and arsines were analyzed through the use of G2 ab initio calculations and the examination of experimental data obtained by means of FT-ICR techniques. The alpha,beta-unsaturated amines, phosphines, and arsines are less basic but significantly more acidic than the corresponding saturated analogues. However, while both vinyl-and ethynylamine protonate preferentially at the beta-carbon atom, vinyl- and ethynylphosphine are phosphorus bases in the gas-phase. Arsines resemble closely the corresponding phosphines, although protonation at the C-alpha atom competes with protonation at the heteroatom. The enhanced acidity of unsaturated compounds can be attributed essentially to a stabilization of the anions because of a favorable interaction of the XH- group with the C-C multiple bonds. This stabilizing effect is maximum for amines and minimum for arsines. The bw relative basicity of these unsaturated compounds results from a destabilization of the protonated species due to unfavorable interactions of the XH3+ group With the C-C pi-system. Protonation at the beta-carbon is strongly favored for amines but unfavorable for phosphines and arsines, because the carbocation formed is much less stabilized when the heteroatom of the XH2 group belongs to the second or the third-row than when it is a first-row atom.