Influences of two different developing methods on the optical parameters of dichromated gelatin (DCG) were studied. These two processes for obtaining DCG plates ready for use have been presented by Meyerhofer and Chang and Leonard. The difference between these methods is that the latter involves a hardening process after exposure. The gelatin layers used were obtained from Kodak 649F photographic plates by removing the silver halide. By using the first method, it was observed that the absorption modulation, and hence also the refractive index modulation, penetrate during exposure through the whole DCG layer. The gratings begin to saturate when the exposing energy exceeds 200 mJ cmm 2. In the second method absorption was low. According to our measurements the modulations can reach only 20–30 per cent of the grating thickness. Because the diffraction efficiency of this hologram can be nearly 100 per cent, the refractive-index modulation is very high.