The involvement of ground and excited state proton transfer reactions in matrix-assisted laser desorption ionization (MALDI) of bradykinin and bovine insulin is examined using a series of p-substituted aniline compounds as matrices. Semiempirical calculations of ground and excited state acidity of the p-substituted aniline and anilinium ions are presented. A linear correlation between log (analyte [A + H](+) ion yield) and matrix acidity is obtained. The behavior of the seven p-substituted anilines is discussed in terms of the relationship between matrix compound structure, reactivity, and ability to act as a MALDI matrix.