In order to overcome uniformity problems in large u.v. holoconcentrators recorded in dichromated gelatin, the structure of hardened gelatin layers has been studied. X-ray diffraction patterns show that layers are formed by individual gelatin chains partly associated as triple-stranded helical rods as those found in native collagen. These rods, in turn, may form fibrillar aggregates. Helical rods and microfibrils are, to a great extent, parallel to the layer surface and their relative amount and packing depend on the hardener as well as on temperature and drying time after coating. X-ray diffraction offers the possibility of estimating in a simple manner the relative amount of triple-helical rods in crystallographic register within the gelatin layers. From these results, a method for obtaining uniform large holograms by means of a slow layer drying process is proposed.