Thiocarbamides were converted to their di-N-pentafluorobenzyl (PFB) derivatives and analysed by gas chromatography/negative-ion chemical-ionization mass spectrometry with methane as reagent gas. The PFB derivatives of the 2-thiouracils gave mass spectra in which the ion current was carried largely by an ion arising from [M-PFB]-. The derivative was used in the determination of the uptake and metabolism of thiocarbamides by cultures of melanoma cells.