A shift of pH of pepsin solutions from 4.6 to 1.0 gives rise to spectral displacements in the ultraviolet. If represented as difference spectra three peaks with maxima at 2770, 2850, and 2930 Angströms are present which can be attributed to the tyrosine and tryptophan residues in the protein. On mild autolysis of pepsin at pH 2.0 the absorbancy in the ultraviolet further decreases. Although some of these effects can be ascribed to the occurrence of hydrogen bonding between the aromatic residues and a carboxylate ion, those observed on autolysis are caused by charge effects of newly formed polar groups in the vicinity of a chromophore. No direct relation between the optical properties described here and enzymic activity of pepsin has been observed.