The gas-phase acidities of methylidynephosphine, HCtbond;P, ethylidynephosphine, CH(3)Ctbond;P, and ethylidynearsine, CH(3)Ctbond;As, have been measured by means of Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry and calculated at the CCSD(T)/6-311+G(3df,2p)//QCISD/ 6-311+G(df,p) level of theory. An analysis of these results shows that, in contrast to the well-known fact that HCtbond;N is a stronger acid than CH(3)Ctbond;N, CH(3)Ctbond;P and CH(3)Ctbond;As are more acidic than HCtbond;P and HCtbond;As, respectively. The most important consequence of this unexpected effect is that while HCtbond;P and HCtbond;As are found to be weaker acids than HCtbond;N, the opposite trend is found for the corresponding methyl derivatives, the acidity of which increases as CH(3)Ctbond;N<CH(3)Ctbond;P<CH(3)Ctbond;As. Also the effects of deprotonation on the structures and the vibrational frequencies of HCtbond;X and CH(3)Ctbond;X (X=N, P, As) compounds are qualitatively similar, but quantitatively very different for nitrogen- as compared with phosphorus- and arsenic-containing compounds. A rationalization of these differences in terms of the bonding differences is presented.