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The holograms with intenferential structure formed by colloidal Ag are wide spread in display holography. The main advantages of these holograms are the high DE running at 50% in wide range of the visual spectra, low noise and durability during an operation.Special high definition photomaterial PFG-02, PFG-03 and the other are used for formation colloidal Ag in the layer. The microcrystals of AgHal have a diameter less than 0.03 nm. This is not enough. If the exposed layer is developed with usual developer, for instance D-19, particles of Ag would be as metallic threads and ribbons. Colloidal Ag is formed in special in very deluted developers.For study of the colloidal Ag formation during developing the investigation of developer components and reaction products accumulation in differently exposed LOI-2 layer was carried out. Fig. 1 showes the change of layer optical density, caused by developing of the layer in fenidon-hydroxynon developer with pH = 9.9±0.1. The curves on fig,2 demonstrated the change of pH, measured by plane glass electrode; and on fig 3 -a potencial of platinum plane electrode, which is characterized by the change of oxident-reduction potential (E) of developing system. Given results were obtained for electrode in the layer depth of 20 m.Analysis of the curves for the similar exposed layers let us to make two major conclusions, which are nessesary for an explanation of holographic developer action: a) pH and E in a layer under developing are than low that less an exposure; for such layer the time before the start of developing is the shorties; b) for intensively exposed layer at increasing of optical density D during developing, E have being decreased too.Taking account of as stated above and the fact that the developer content can be considered as the same in exposed and unexposed plots of high frequency hologram structure due their little sizes, it is possible to suggest the following explanation of mechanism formation of colloidal Ag in holographic developers. The reduction of the most exposed AgHal microcrystals is began at once when the layer is soaked in the developer. The process is began at the lowest pH. Fig.3, Change of ox/red potential on 2Oum depth of LOI-2 layer during developing. Developer: fenidone-hydroxenon pH=9.9±0. 1. Exposure: 1 -H=O (fog) , 2 -H3.8x104 luxxsec, 3 -H6.OxlO4luxxsec, 4 -H1.0x105 luxxsec. 192