Fragment ion-based proteome quantification methods quantify peptides based on unique peptide fragment ions avoiding the issue of ratio distortion that is commonly observed for reporter ion-based quantification approaches. Herein, we present a novel approach that relies on a collision-induced dissociation (CID)-cleavable isobaric acetyl-isoleucine-proline-glycine (Ac-IPG) tag, which conserves the merits of quantifying peptides based on unique fragment ions while reducing the complexity of the b-ion series thus facilitating data processing. Multiplex labelling is based on selective N-terminal dimethylation followed by derivatizing the C-terminal Lys residue of LysC peptides with isobaric Ac-IPG tags with different isotope distributions on Pro-Gly and Ac-Ile. Upon fragmentation between Ile and Pro, the resulting y-ions with the neutral loss of Ac-Ile can be distinguished between the different labelling channels based on different numbers of isotope labels on the Pro-Gly part. The peak intensities of y-ions contain the information for relative quantification.