Phthalic acid esters are among the most frequently encountered pollutants in biological extracts. They are very often detected in biological analysis using gas chromatography. Even using high resolution glass capillary columns, the identification of some of these esters remains difficult. In this paper the mass spectral fragmentation of a large number of phthalic acid esters is presented. Some criteria for the identification of the type of isomerism on the aromatic ring and the identification of the ester functions are also presented.