The reaction of NN′-dimethylimidazolidine-2-thione with methyliodide followed simple second-order kinetics in polar solvents, whereas in less polar solvents the reverse process made a significant contribution to the overall rate. The solvent effects on the forward rate constant were linearly correlated with those on the rate of the reaction of tetramethylthiourea with methyliodide. However, the reactions