The unsubstituted phosphirenylium ion, cyclo-C 2H 2P +, was generated in the gas phase via an addition-ablation strategy employing a modified quadrupole ion-trap mass spectrometer. The reaction of gas-phase PBr + with acetylene affords H 2C 2P + as the sole product ion. Its structure is that of the phosphirenylium cation (1). Experimental results find support from density functional theory calculations at the B3LYP/6-31G* and G3//B3LYP/6-31G* levels. The reaction of PBr + with acetylene is predicted to be exothermic with respect to the formation of two isomers, the phosphirenylium cation (1) and the ethenylidenephosphenium cation (2), whereas formation of eight other isomers is endothermic. Reactions of 1 with acetylene and ethylene do not lead to observable products, but 1 does react with 1,3-butadiene as a phosphenium ion via formal 1,4-addition to give the 3-phosphaspiro[2,4]-hepta-l,5-diene phosphonium ion. The formation of this P-spiro adduct is predicted to be a stepwise process. An isotopic labeling experiment supports this structure, which is predicted by computational modeling to be kinetically favored over a lower energy structure that could arise from a Diels-Alder-like reaction of the phosphirenylium ion.