In an attempt to extend the application of the phenylisothiocyanate degradation of peptides it was found necessary to study the kinetics of the conversion of phenylthiocarbamyl amino acids into phenylthiohydantoins. The conversion was found to obey first-order kinetics and to be catalyzed by hydrogen ions. A set of conditions with regard to time, hydrogen ion concentration and temperature was found, which allowed the quantitative or near quantitative conversion of all phenylthiocarbamyl amino acids into phenylthiohydantoins with the only exception of the phenylthiohydantoin of serine, which was returned in a yield of 20%.