The rate of hydrolysis of 2-methyl-Δ2-thiazoline has been studied as a function of pH, in media ranging from concentrated HCl to buffer solutions at pH 9. The reaction was followed by measurement of ultraviolet absorption at 260 mμ and also by Raman spectroscopy, The rate shows a maximum at about pH 3, the molecule being quite stable in concentrated HCl and hydrolyzing very slowly in neutral solutions. A reaction scheme is proposed, assuming a hydroxythiazolidine as an intermediate, to explain the observed results, including the appearance of both N- and S-acetyl-β-mercaptoethylamines. Studies on the reactions of the two latter compounds in solution at various pH values gave results concordant with the scheme. Velocity constants for most of the processes involved in the scheme are recorded. The pKa value of the conjugate acid of 2-methylthiazoline was found to be 5.22±0.02 at ionic strength 0.10. The reactions of N-acetyl-β-mercaptoethylamine and of glutathione, to yield thiazoline rings or thiol esters, are compared. It is concluded that the open chain N-acetyl form is thermodynamically stable in neutral solution relative to the thiazoline, although the hydrolysis of the latter is very slow near pH 7 in solutions of 2-methyl-Δ2-thiazoline. The implications of the observations for the hypothesis of thiazoline formation from cysteinyl residues in proteins are discussed.