A useful thin‐film passive dosimeter system has been developed through use of a poly(halo)styrene matrix doped with malachite green methoxide as an indicator. Extensive characterization studies of the chlorostyrene dosimeters have been completed and the range of conditions under which they are useful has been defined. Exposure to radiation causes development of a blue color (absorption at 630 and 430 nm) which has an optical density proportional to dose over the range 104−107rad and may be calibrated to above 3×107rad. No effects of dose rate have been observed between 102and 1015rad/sec, and no effects of photon energy between 2 keV x‐rays and 1 MeV gamma rays. Of the environmental parameters studied, only extraneous uv light, temperature, and a few noxious gases have a significant effect on the performance of the dosimeters. Oxygen has only a small effect (a few percent) at low dose rates, but acidic gases such as NO2, SO2, and HCl will first color, then bleach the films. The rate and ultimate amount of coloration are dependent on temperature, but this effect is negligible from 0 to 35°C. These films have an indefinite shelf life at room temperature in the absence of light and corrosive gases, and the small effects of oxygen and temperature can be compensated for in the calibration for a particular application.