Positive ion mass spectra were obtained from several coumarin oral anticoagulants (phenprocoumon, warfarin, acenocoumarol and dicoumarol) and derivatives by liquid chromatography-thermospray mass spectrometry (LC-TSP-MS) and liquid chromatography-electron impact mass spectrometry (LC-EI-MS) to assess the use of LC-MS methods for the determination of these compounds in biological materials. LC-TSP mass spectra showed a single [M + 1]+ ion with no fragmentation; LC-EI mass spectra showed fragment ions which were similar in mass and relative intensities to those obtained by conventional EI-MS. These data should serve as a basis for the development of LC-MS methods for the qualitative and quantitative analysis of coumarin anticoagulants in biological samples. LC-TSP-MS was applied to the determination of phenprocoumon in a plasma extract from an anticoagulated patient.