The use of amino thioacids in the 'azirine/oxazolone method' led to completely epimerized Aib-containing endothiodipeptides (Aib = 2-aminoisobutyric acid). It could be established that the epimerization occurred during the acidic hydrolysis of the primarily formed dipeptide thioanilides in which the thiocarbonyl group was shifted from the last to the penultimate amino-acid residue. Several conditions for the hydrolysis were tested, and, in some of them, the degree of epimerization could be reduced. By treatment of the Aib-containing dipeptide thioanilides 21 with ZnCl2 in AcOH followed by HCl in AcOH, the isomeric endothiodipeptide anilides 25 were formed, i.e., the thiocarbonyl group was again shifted from the last to the penultimate amino acid residue. Under optimized reaction conditions, this novel isomerization proceeded in high yields and without any epimerization. Two conceivable mechanisms are proposed in Scheme 12. X-Ray diffraction analyses were performed for Z-Gly-Aib-Y(CS)-N(Me)Ph (21f) and the isomeric Z-Gly-Y(CS)-Aib-N(Me)Ph (25f).