Our investigations on the reaction mechanism to account for regioselectivity on the addition of indoles to unsymmetrical silyloxyallyl cations are reported. Using both experimental and computational methods, we confirmed the significance of steric effects from the silyl ether group toward directing the inward approach of indoles, leading to nucleophilic attack at the less substituted electrophilic α'-carbon. The role of residual water toward accelerating the rate of reaction is established through stabilization of the participating silyloxyallyl cation.