Phototransformations involved in dichromated poly(acrylic acid) (DCPAA) were analyzed under irradiation conditions representative of hologram formation. It was shown that, in parallel to the reduction of chromium(VI) into chromium(III), the crosslinking of the matrix occurred through formation of covalent bonds between macromolecular chains. A comparison with dichromated poly(vinyl alcohol) (DCPVA) irradiated under the same conditions was also reported giving evidence for the crucial role played by the chemical structure of the polymeric matrix in the hologram formation. The combined approach based on the fate of both the absorbing species, chromium(VI) and the polymer allowed to completely elucidate the reactions taking place upon irradiation.