Abstract: A monitoring method for application in solid phase peptide synthesis. In one aspect, the synthesis starts with an amino acid residue protected by an N-alpha-amino protecting group and involves the steps: (a) removing the N-alpha-amino protecting group to obtain an N-alpha-amino group, (b) adding an amino acid residue protected by an N-alpha-amino protecting group, via a peptide bond, to the N-alpha-amino group obtained in step (a) by use of a reactive protected amio acid derivative, and (c) repeating steps (a) and (b). The reaction system includes 3-hydroxy-1,2,3-benzotriazin-4(3H)-one or a derivative thereof; and the color of the reaction system or of a component thereof is monitored during the synthesis.