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The reversible Smiles rearrangement between S-(2,4-dinitrophenyl)cysteine (la) and N-(2,4-dinitrophenyl)cysteine (2a) has been kinetically studied in methanol, DMF, and Me^O containing an organic base such as imidazole or DBU. The overall reaction is composed of the spontaneous and base-catalyzed reactions. In methanol the rearrangement from la to 2a goes virtually to completion in most cases. In DMF containing DBU, the reverse rearrangement from 2a to la becomes observable to afford eventually an equilibrium mixture of la and 2a. During this transformation an intermediate was spectroscopically detected, which showed an absorption band around 500 nm. The amide derivative of la (lb) failed to undergo a facile rearrangement except under much severer