The phosphopeptide standard mixture is formulated for MALDI-MS. It contains equimolar amounts of three unphosphorylated and four phosphorylated peptides, ranging in mass from 1046 to 2192 and representing phosphoserine (pS), phosphothreonine (pT), and phosphotyrosine (pY) monophosphopeptides as well as a peptide containing both pT and pY. This mixture is ideal for use as an internal or external control for LC/MS, MALDI analysis, or b-elimination reactions.