A prototype x-ray system based on a liquid-metal-jet anode was evaluated within the framework of the LABSYNC project. The generated spectrum was measured using a CZT-based spectrometer and was compared with spectra simulated by three Monte Carlo codes: MCNPX, PENELOPE and EGS5. Notable differences in the simulated spectra were found. These are mainly attributable to differences in the models adopted for the electron-impact ionization cross section. The simulation that more closely reproduces the experimentally measured spectrum was provided by PENELOPE. •The x-ray spectrum of a liquid-jet x-ray anode was measured with a CZT spectrometer.•Results were compared with Monte Carlo simulations using MCNPX, PENELOPE, EGS5.•Notable differences were found among the Monte Carlo simulated spectra.•The key role was played by the electron-impact ionization cross-section model used.•The experimentally measured spectrum was closely reproduced by the PENELOPE code.