The sulfoxide glycosylation reaction works remarkably well for many difficult glycosylations. We attribute this in pari to the fact that an extremely reactive intermediate can be generated rapidly under mild conditions at low temperature. We find that anomeric sulfenates can be formed in the reaction and that these intermediates impede glycosylation at low temperature. A mechanism for sulfenate formation is proposed and a strategy for minimizing sulfenate formation is presented. The energetics and reactivity of anomeric sulfenates are also investigated. The mechanistic investigations described below have implications for other glycosylation reactions as well.