A new amino protecting group, the methylsulfonylethyloxycarbonyl (Msc) group, is described which combines well with other familiar groups (benzyloxycarbonyl, t‐butyloxycarbonyl) in peptide syntheses. Its main characteristics are an extreme acid stability, a high base lability and a high polarity which enhances solubility in polar solvents including water. The group resists catalytic hydrogenolysis but does not poison the catalyst. It has good crystallizing properties. Application in peptide synthesis is exemplified in the synthesis of Msc‐Phe‐Arg‐Trp‐Gly‐OMe.HCl. Deprotection of the masked tetrapeptide was accomplished within 5 sec with a 1.0 N solution of base (OH‐and OCH3‐). A reaction scheme for the cleavage of the gro