A new method for using dichromated gelatin films in photolithography has been recently developed in our laboratory. In this paper, we describe the experiments carried out for making efficient surface-relief diffraction gratings using that method. Basically, gratings are obtained by developing, in an oxygen electron cyclotron resonance stream, the interference of two coherent waves recorded on a dichromated gelatin film. Diffraction efficiency and incident angle selectivity of 0.5 mu m period gratings are studied as a function of development time, and results are compared with theoretical predictions.