The effects of thiomethoxy and methoxy substituents on the rates and the mechanisms of anionic oxy-Cope rearrangements were investigated with density functional theory (B3LYP/6-31+G*), Thiomethoxy substituents at positions C4 or C6 of 3-oxido-1,5-hexadienes cause acceleration and promote heterolytic cleavage rather than concerted rearrangement, and the Cope product is formed via a stepwise pathway. The corresponding methoxy substituents impede heterolytic cleavage and slow the concerted rearrangement.