The gas-phase basicities and acidities of ethynyl-, vinyl-(or ethenyl-), and ethylarsine have been measured by means of FT-ICR techniques. The structures of the different neutral, protonated, and deprotonated species were elucidated by means of ab initio molecular orbital calculations at the MP2 level. Their final energies were calculated in the framework of the G2 theory. The agreement between calculated and experimental values for both basicities and acidities was fairly good. Protonation of vinylarsine and ethynylarsine at the C-alpha carbon leads to the breaking of the C-As bond, such that the corresponding protonated species can be viewed as complexes between ethylene and acetylene, respectively, with a AsH2+ cation. More importantly, these complexes are predicted to be as stable as the cations formed by direct protonation at the arsenic atom. For the particular case of ethynylarsine, this result is consistent with the experimental evidence. The gas-phase basicity decreases (GB values decrease) and acidity increases (Delta(acid)G(0) values decrease) in the following order: ethylarsine, vinylarsine, ethynylarsine. The performance of the B3LYP density functional approach in the description of the thermochemical properties of these arsenic-containing species was also studied.