Ab initio molecular orbital calculations are reported for closed-shell organosilicon cationic species SiC2Hn+ (n = 1, 3, 5). Gradient optimizations have been performed on all minima and on all transition structures with the standard Gaussian split-valence 6-31G(d,p) basis set, both at the SCF level of theory and with inclusion of electron correlation to second-order Moller-Plesset (MP2) perturbation theory. Several critical points, including those which correspond to the global minimum on each surface, also have been optimized at MP2-(full)/6-311G(d,p); all other critical points have been calculated at MP2(full)/6-311G(d,p)//MP2(full)/6-31G-(d,p). Each critical point has been characterized by harmonic frequency calculations at the SCF/6-31G(d,p) level of theory, from which, also, zero-point vibrational energies (ZPE) were obtained for each species. Furthermore, the structure at the global minimum on each surface at MP2(full)/6-31G(d,p) was characterized by a harmonic frequency calculation. Vibrational frequencies, as well as zero-point vibrational energies, also are reported at this level for these compounds. In general, for ions SiC2Hn+ structures having all the hydrogen atoms attached to carbon have the lowest energies; the only exception is for the SiC2H5+ surface on which the 1-silaallyl cation, H2SiCH=CH2+, is at the global minimum, 3.9 kcal mol(-1) below CH3CH2Si+. Large isomerization barriers have been calculated for the interconversions of some low-energy ions. The existence of such barriers suggests the presence of more than one stable isomer on each surface in the gas-phase.