A two-dimensional position-sensitive dosimetry system based on a scintillating gas detector is being developed for pre-treatment verification of dose distributions in particle therapy. The dosimetry system consists of a chamber filled with an Ar/CF4 scintillating gas mixture, inside which two gas electron multiplier (GEM) structures are mounted (Seravalli et al 2008b Med. Phys. Biol. 53 4651–65). Photons emitted by the excited Ar/CF4 gas molecules during the gas multiplication in the GEM holes are detected by a mirror–lens–CCD camera system. The intensity distribution of the measured light spot is proportional to the 2D dose distribution. In this work, we report on the characterization of the scintillating GEM detector in terms of those properties that are of particular importance in relative dose measurements, e.g. response reproducibility, dose dependence, dose rate dependence, spatial and time response, field size dependence, response uniformity. The experiments were performed in a 150 MeV proton beam. We found that the detector response is very stable for measurements performed in succession (? = 0.6%) and its response reproducibility over 2 days is about 5%. The detector response was found to be linear with the dose in the range 0.05–19 Gy. No dose rate effects were observed between 1 and 16 Gy min?1 at the shallow depth of a water phantom and 2 and 38 Gy min?1 at the Bragg peak depth. No field size effects were observed in the range 120–3850 mm2. A signal rise and fall time of 2 ?s was recorded and a spatial response of ?1 mm was measured.