The rates of base-catalyzed protium–deuterium exchange (α-thioenolization) of the exo and endo protons of thiocamphor (1), in 2:1 dioxane-D2O at 25.0 ± 0.5 °C, have been determined by monitoring the uptake of deuterium mass spectrometrically. The exo and endo exchange rate constants, 2.20 × 10−2 and 5.60 × 10−4 M−1 s−1, respectively, are 23.2 and 12.3 times larger than the rate constants for exo and endo exchange in camphor (2). Factors which may determine the rate enhancements are discussed.