An ab initio theoretical study of the (SiC2H2)(+) species has beeen carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. A cyclic SiC2H2+(B-2(2)) state has been shown to be the global minimum; however, a vinylidene-type structure, SiCCH2+(B-2(2)), has been found to lie very close in energy (2.4 kcal/mol higher than SiC2H2+(B-2(2)) at the PMP4 level). Following in energy are a cyclic SiC2H2+((2)A(1)) isomer and the insertion product HSiCCH+((2)A’), which lie about 9 and 14.7 kcal/mol, respectively, above the cyclic B-2(2) state. Another isomer, H2SiCC+, is shown to be quite unstable. The reaction of Si+ with acetylene to give SiC2+ + H-2 is found to be clearly endothermic, whereas the production of SiCCH+ seems to be only slightly exothermic. In addition, we have found a channel for the production of SiCCH+ which seems to proceed without activation energy. Therefore, according to our theoretical results, the reaction of Si+ with acetylene could take place under low-temperature conditions, such as in interstellar media.