The positive-and negative-ion chemical ionization ( k CI) behavior of phthalic acid and four related compounds (phthalic anhydride, phthalide, biphthalyl, and anthraquinone-2-carboxylic acid) are characterized by tandem mass spectrometry with lowenergy collisionally activated dissociation. Principal fragmentation pathways have been established, and molecular and fragment ions arising from these compounds have been differentiated from adduct and dimer ions, as well as from ions due to impurities. The ubiquitous nature of these compounds in analytical samples makes it important to identify the corresponding ions, and thus to be able to exclude them from consideration in the identification of other sample components.