Structure analysis of dialkyl propylphosphonates requires the specific identity of isomeric propyl groups bonded to phosphorus.To this end,homologous series of isomeric dialkyl iso-and n-propylphosphonates were analysed by gas chromatography-quadrupole tandem mass spectrometry.For identification of the isomer,characteristic ion m/z 125 in electron impact mass spectrometry（EI-MS） and quasi-molecular ions in chemical ionization mass spectrometry（CI-MS） were studied by tandem mass spectrometry（MS/MS） in product scan mode.The parameters including collision energy and collision gas pressure were optimized.The experiments showed that precursor ions m/z 125 of dialkyl n-propylphosphonate screened mainly product ion m/z 107 under condition of 20 V of collision energy and 1.0 mTorr of collision gas pressure by EI-MS/MS,while precursor ions m/z 125 of dialkyl isopropylphosphonate screened mainly product ions m/z 65 and m/z 83.In the same condition,quasi-molecular ions of dialkyl n-propylphosphonate,which were precursor ions,screened mainly product ions m/z 125 and 107 and quasi-molecular ions of dialkyl isopropylphosphonate screened mainly product ions m/z 125,107 and 83.For most chemicals,the product ion m/z 125 is base peak.Based on EI and CI tandem mass spectrometry,the isomeric dialkyl propylphosphonates could be rapidly identificated.