The use of amino thio S-acids in the 'azirine/oxazolone method' and a novel isomerization led to Aib-containing endothiopeptides. With the aim of generalizing this method, a variety of Aib-containing dipeptide thioanilides have been prepared. By their treatment with ZnCl2 in AcOH, followed by HCl-saturated AcOH, the C=S group was shifted from the last to the penultimate amino acid in high yield and without epimerization. As this methodology is very useful for the specific introduction of a thioamide group, it was extended to Aib-containing tripeptides. In addition, it could be shown that a mechanism via spirocyclic intermediates (cf. Scheme 4) is most likely for this isomerization. To establish the proposed neighboring-group participation of the N-acyl group, model dipeptide thioanilides containing no N-terminal C=O group were synthesized. These derivatives did not undergo rearrangement.