When MS is used to analyze a tryptic digest of a protein, when the structural questions being pursued are related to phosphorylation, MALDI offers a simple approach. The digestion mixture can be analyzed directly. Unfortunately, phosphorylated peptides exhibit a response which is at least an order of magnitude less than that for nonphosphorylated peptides. We have found that the addition of polyamines and compounds as simple as diammonium citrate can enhance the response for phosphorylated peptides, to yield the most intense peaks in the spectrum. This greatly facilitates phosphorylation studies, and yields peaks sufficiently intense, for analyte levels less than 500 fmol, such that PSD spectra can be obtained.