A direct oxazoline→thiazoline conversion can be realized by thiolysis of oxazolines with H 2S in methanol/triethylamine, followed by cyclodehydration with Burgess reagent. This protocol is high-yielding, chemoselective, and essentially free of racemization for C(5)-unsubstituted and trans-4,5-disubstituted peptide oxazolines. Thioamide intermediates are obtained regioselectively, thus the thiolysis of oxazolines offers an alternative to the thiation of peptides with Lawesson's reagent.