The kinetics of the base-catalysed H/D exchange of the 1-methylthiolanium cation in heavy water has been studied. Of the four α-methylene protons, two exchange much faster than the other two, a result which requires that reprotonation of the carbanion intermediate occurs with retention of configuration. NMR. spectral analysis indicates that the protons undergoing faster exchange bear a trans relation to the lone-pair on sulfur. This conclusion appears to conform to Wolfe's theory on the stability of carbanions [3].