An aminooxy-containing peptide, the nucleophile partner for oxime ligations, is usually grafted on a NH[2]-peptide resin by activating a protected aminooxyacetic acid as an active ester. Here, we have shown that its subsequent coupling to NH[2]-peptide resin competes with the overacylation of the -NH-O- nitrogen and that the overacylation level increases with the basicity of the reaction mixture. Moreover, we found that overacylation is prevented when the COOH of the Aoa-derivatives is engaged in an amide bond.