The structures of all plausible isomers of the SiC2H7+ ion were optimized at the SCF and correlated levels of theory, including the MP2/6-31G** and the B3LYP/6-31G** methods. At all levels of theory the most stable isomer was found to be (CH3)2SiH+ (1). At the SCF level the next was C2H5SiH2+ (2), lying ca. 21 kcal/mol higher, followed by the symmetric SiH3-bridged H3SiCH2CH2+, which can also be regarded as an addition/elimination complex of SiH3+ and C2H4 (5 is ca. 24 kcal/mol higher than 1). Two other isomers, CH3SiH2CH2+ (3) and CH3SiH3CH+ (4) lie significantly higher. However, at the B3LYP/6-31G** level 3 is no longer a stationary point. No transition state was found for the direct interconversion of 1 and 2. It is proposed that this experimentally observed interconversion occurs through 5. The corresponding transition state (TS1) has a Cs symmetry and corresponds to the symmetric shift of two hydrogens. At the SCF levels it lies higher than SiH3+ + C2H4, but at correlated levels it becomes several kcal/mol lower, thus allowing isomerization before dissociation. Other transition states connecting 5 with 2 through 4 lie lower than TS1 at all levels of theory. The symmetric nature of the TS1 transition state may account for the observed differences in the exchange reactions of CH3SiH2+ and (CH3)2SiH+ with deuterated ethene.