An ab initio molecular orbital study of the PC2H2+ species has been carried out. Geometries and harmonic vibrational frequencies have been computed at the MP2/6-31G* level. On the singlet surface, cyclic PC2H2+- ((1)A(1)) is found to be the global minimum, in agreement with previous theoretical studies. On the other hand, the lowest-lying triplet state is PCCH2+((3)A(2)), With the pi-complex PC2H2+((3)A(2)) and HPCCH+((3)A ") lying 7.3 and 17.7 kcal/mol, respectively, higher at the PMP4 level of theory. The reaction of pf(P-3) with acetylene is clearly endothermic for the production of C2P+, whereas formation of PCCH+ is exothermic (-10.7 kcal/mol at the G1 level). In addition, we have found at least one barrier-free channel leading to PCCH+, which implies insertion of P+ into a C-H bond, followed by elimination of the hydrogen bonded to phosphorus. A second mechanism, which involves isomerization into PCCH2+((3)A(2)), is also predicted to proceed without activation energy, or in any case a very small one. Therefore, the reaction of P+ with C2H2 could take place in interstellar media, and PCCH+ may be a precursor of interstellar C2P. The latter is predicted to have a high proton affinity, about 214 kcal/mol.