An interface between a capillary supercritical fluid chromatograph and a double-focusing mass spectrometer was developed. Modification of the standard electron ionization (EI)-chemical ionization (CI) combination ion source was necessary to obtain useful mass spectra with negative-ion detection. A detection limit in the lower nanogram range of the chlorinated pesticides (DDT and dieldrin) was found irrespective of the mode of detection. Positive-ion methane CI resulted in a relatively abundant [M+ H] + ion, whereas positive-ion isobutane and ammonia CI appeared not to be amenable to the detection of chlorinated pesticides. The EI-charge exchange mass spectra of the investigated pesticides generally did not match the library mass spectra. In the negative-ion mode, CO 2 was an efficient moderating gas giving relatively large amounts of M −·, in addition to some fragment ions. More fragmentation was observed when N 2O replaced CO 2 as the mobile phase. No major effects on the mass spectra, obtained by using pure mobile phase, were observed on adding methane, isobutane or ammonia.