A systematic bias has been observed in the calibration ofa number of commercial quadrupole gas chromatographic/mass spectrometric (GC/MS) instruments using conventional internal standard quantification techniques. Response factors between deuterated polynuclear aromatichydrocarbons and selected analytes are presented for aseries of calibration curves generated according to commonly used capillary column semivolatile organic environmental analysis methods. Examples of pronouncedresponse factor trends are given over calibration rangesof <1 order of magnitude. Data are gathered over thecourse of several years from mass spectrometric systemsof differing manufacturer and model. A need toexamineand carefully utilize GC/MS calibration curves beyondcommon practice is demonstrated.