AbstractThe side reactions observed on acid hydrolysis (6NHCl; 20 hrs at 115°C) of carbobenzoxy derivatives of tyrosine containing peptides were investigated by hydrolyzing carbobenzoxy‐L‐tyrosine or O‐benzyl‐L‐tyrosine under the same conditions or by reactingL‐tyrosine with benzylchloride in the presence of 6N HCl. In every instance the reaction mixture contained besides tyrosine 3 new products distinguishable by paper chromatography. The fractionation of the mixture yielded a mono‐benzylated tyrosine considered to be 3‐benzyl‐L‐tyrosine (I) and two different products which were shown to contain 2–3 benzyl residues in the aromatic ring. These results demonstrate that the benzyl cation formed in the course of the hydrolysis of carbobenzoxy derivatives of tyrosine containing peptides transformsL‐tyrosine into derivatives substituted