Infrared analysis has shown that acid-catalyzed hydration of ethylthioethyne in heavy water does not involve the uptake of deuterium in the initial compound. This confirms earlier evidence that protonation of the triple bond is the ratedetermining step.Values for the entropy of activation of five I-alkynyl thioethers were determined. They range between -1 and -6 e.u. The rate of hydration of 1-tert.-butylthio-I-propyne in 0-98.7 weight-percent ethanol-water mixtures has been analyzed by a theory of Grunwald. A plot of (log ki-log fH@) against Y O gives a straight line. The triple bond of vinylthioethyne has been hydrated in moderately concentrated perchloric acid solutions. A Zucker-Hammeff plot of these data affords a straight line with a slope of 1.07 and a Bunneft plot a "straight" line with a slope of -0.9. These facts suggest that the addition of the water molecule takes place in a step subsequent to protonation.