Experimentally determined barriers to O-acyl group topomerization in mixed anhydrides composed of β-disubstituted carboxylic acids and cyclic thiohydroxamic acid N-hydroxy-4-methylthiazole-2(3H)-thione were located in the range of ΔG‡320=68±8 kJ mol−1 (DNMR). According to modeling studies, the underlying exchange process is proposed to occur via rotation about the N,O bond for torsional movement of the O-acyl