A reinvestigation of the stepwise degradation of peptides and proteins by the Schlack and Kumpf (1926) procedure, which involves conversion of the terminal carboxyl residue to a 5-substituted-2-thiohydantoin; has been undertaken. The following amino acids or their aoetyl derivatives have been converted by warming with acetic anhydride and ammonium thiocyanate into the corresponding 1-acetyl-2-thio-hydantoins: valine, methionine, isoleucine, α-aminophenylacetic acid, tryptophane, histidine, and tyrosine. These new 1-acetyl-2-thiohydantoins, as well as others, have been deacetylated by an improved procedure. Certain 1-acyl-5-carbethoxy-2-thio- hydantoins on hydrolysis give 2-thiohydantoin only. N-acetylthreonine, acetic anhydride, and ammonium thiocyanate produce 5-ethylidene-2-thiohydantoin, also obtained by the condensation of acetaldehyde with 2-thiohydantoin or its 1-acyl derivatives, or from the interaction of ammonium thiocyanate with 2-phenyl-4-ethylideneoxazol-5-one. This type of rearrangement occurs in other cases.