A set of mass labels, wherein each mass label comprises the formula: X-L-M-Re wherein: - X is a reporter moiety having an exact mass, - L is a bond cleavable by collision in a mass spectrometer, - M is a mass modifier, and - Re is a) a reactive functionality for attaching the mass label to an analyte or b) the analyte, wherein each mass label in the set has an integer mass, wherein each mass label in the set has the same integer mass, wherein the set comprises two or more subsets of mass labels, each subset comprising one, two or more mass labels, wherein, when the subset comprises two or more mass labels, the exact mass of the reporter moiety X of each mass label in the subset is different from the exact mass of the reporter moiety X of the other mass labels in the same subset and in all other subsets, wherein each mass label is distinguishable by mass spectrometry, and wherein each mass label has the following structure: wherein * represents that oxygen is 18 O, carbon is 13 C, nitrogen is 15 N or hydrogen is 2 H, and one or more * may be present; provided that the set comprises n = 24 mass labels having the following structures: TMT-5-24-127.12297 (Subset 1) TMT-5-24-128.12001 (Subset 2) TMT-5-24-128.12633 (Subset 2) TMT-5-24-129.11704 (Subset 3) TMT-5-24-129.12336 (Subset 3) TMT-5-24-129.12968 (Subset 3) TMT-5-24-130.1204 (Subset 4) TMT-5-24-130.12672 (Subset 4) TMT-5-24-130.13304 (Subset 4) TMT-5-24-131.12375 (Subset 5) TMT-5-24-131.13007 (Subset 5) TMT-5-24-131.13639 (Subset 5) TMT-5-24-132.12711 (Subset 6) TMT-5-24-132.13343 (Subset 6) TMT-5-24-132.13975 (Subset 6) TMT-5-24-133.13046 (Subset 7) TMT-5-24-133.13678 (Subset 7) TMT-5-24-133.1431 (Subset 7) TMT-5-24-134.13382 (Subset 8) TMT-5-24-134.14014 (Subset 8) TMT-5-24-134.1523 (Subset 8) TMT-5-24-135.13717 (Subset 9) TMT-5-24-135.14349 (Subset 9) TMT-5-24-136.14053 (Subset 10) - X is a reporter moiety having an exact mass, - L is a bond cleavable by collision in a mass spectrometer, - M is a mass modifier, and - Re is a) a reactive functionality for attaching the mass label to an analyte or b) the analyte, wherein each mass label in the set has the same integer mass, wherein the set comprises two or more subsets of mass labels, each subset comprising one, two or more mass labels, wherein, when the subset comprises two or more mass labels, the exact mass of the reporter moiety X of each mass label in the subset is different from the exact mass of the reporter moiety X of the other mass labels in the same subset and in all other subsets, wherein each mass label is distinguishable by mass spectrometry, and wherein each mass label has the following structure: wherein * represents that oxygen is 18 O, carbon is 13 C, nitrogen is 15 N or hydrogen is 2 H, and one or more * may be present; provided that the set comprises n = 24 mass labels having the following structures: TMT-5-24-127.12297 (Subset 1) TMT-5-24-128.12001 (Subset 2) TMT-5-24-128.12633 (Subset 2) TMT-5-24-129.11704 (Subset 3) TMT-5-24-129.12336 (Subset 3) TMT-5-24-129.12968 (Subset 3) TMT-5-24-130.1204 (Subset 4) TMT-5-24-130.12672 (Subset 4) TMT-5-24-130.13304 (Subset 4) TMT-5-24-131.12375 (Subset 5) TMT-5-24-131.13007 (Subset 5) TMT-5-24-131.13639 (Subset 5) TMT-5-24-132.12711 (Subset 6) TMT-5-24-132.13343 (Subset 6) TMT-5-24-132.13975 (Subset 6) TMT-5-24-133.13046 (Subset 7) TMT-5-24-133.13678 (Subset 7) TMT-5-24-133.1431 (Subset 7) TMT-5-24-134.13382 (Subset 8) TMT-5-24-134.14014 (Subset 8) TMT-5-24-134.1523 (Subset 8) TMT-5-24-135.13717 (Subset 9) TMT-5-24-135.14349 (Subset 9) TMT-5-24-136.14053 (Subset 10) TMT-5-24-127.12297 (Subset 1) TMT-5-24-128.12001 (Subset 2) TMT-5-24-128.12633 (Subset 2) TMT-5-24-129.11704 (Subset 3) TMT-5-24-129.12336 (Subset 3) TMT-5-24-129.12968 (Subset 3) TMT-5-24-130.1204 (Subset 4) TMT-5-24-130.12672 (Subset 4) TMT-5-24-130.13304 (Subset 4) TMT-5-24-131.12375 (Subset 5) TMT-5-24-131.13007 (Subset 5) TMT-5-24-131.13639 (Subset 5) TMT-5-24-132.12711 (Subset 6) TMT-5-24-132.13343 (Subset 6) TMT-5-24-132.13975 (Subset 6) TMT-5-24-133.13046 (Subset 7) TMT-5-24-133.13678 (Subset 7) TMT-5-24-133.1431 (Subset 7) TMT-5-24-134.13382 (Subset 8) TMT-5-24-134.14014 (Subset 8) TMT-5-24-134.1523 (Subset 8) TMT-5-24-135.13717 (Subset 9) TMT-5-24-135.14349 (Subset 9) TMT-5-24-136.14053 (Subset 10)