Purpose(s): Radiosensitizers are commonly used in radiotherapy in Denmark (following the DAHANCA 5 clinical trail) to enhance tumour control of radioresistant hypoxic tumours. Radiosensitizers implemented particularly in the treatment of hypoxic cells are called ‘electron-affinic’ radiosensitizers. A number of nitroimidazoles and related compounds were studied in vitro as well as in vivo, partially also with high-LET beams with encouraging results. However, the detailed mechanism of the actual radiosensitization is still unknown. The work presented here investigates the fragmentation of nimorazole using mass spectrometry. Understanding the fragmentation of radiosensitizers is crucial in evaluating the radiosensitization potential and developing new and more effective drugs, which may improve TCP in hypoxic tumours when using ion beams such as carbon-12 along with LET-painting techniques. Method(s): Fragmentation experiments have been performed using a Finnigan- LTQ-FT mass spectrometer equipped with an electrospray ionisation source. Collision-induced dissociation (CID) and electron-induced dissociation (EID) have been carried out by mass selecting the desired ions and subjecting them to activation energy in the linear trap and to free electrons of 25.8 eV in the FT-ICR cell. We have examined CID and EID of protonated nimorazole and CID of nimorazole radical anion. Result(s): Electrospray ionisation of a nimorazole in positive mode readily forms the protonated nimorazole [M+H]+. CID spectrum of [M+H]+ (Figure 1a) shows the major fragment to be an ion of m/z 114, which corresponds to the loss of the nitroimidazole ring. Although this fragment is also observed in the EID spectrum (Figure 1b), the dominant fragment here is m/z 110, due to the loss of nitroimidazole ring and CH2 group. In both spectra, a minor loss of NO2 is present. In the case of negative ion electrospray, a radical anion of nimorazole M.- can be formed. CID of M.- generates amongst others the negatively charged nitroimidazole ring and a peak corresponding to a minor loss of NO. Conclusion(s): The fragmentation of nimorazole seems to depend on its charge state. This detailed gas-phase study will unlock its fragmentation mechanism and thus allow future potential for structural improvement.