Here, we present preliminary results on the grating formation mechanism in CA-PEG sensitized with new BODIPYs and reference BDPI2. Our approach is based on the evaluation of the main contributor to the grating formation by applying UV-post exposure with uniform intensity as was previously demonstrated for the acrylamide-based photopolymer containing Erythrosine B and NPG as photoinitiators. UV post-exposure (after holographic recording) induces changes in the refractive index modulation due to further bond conversion evenly through the thickness. Changes in the refractive index modulation can be estimated by evaluating the changes in the diffraction efficiency as these parameters are correlated according to the coupled wave theory as described in section 10.2 of the Supporting Information. 57 Gratings recorded on 80±5 m thick layers (1b, 1d and BDPI2) using the exposure energy of 4 J cm -2 were exposed to UVA light with a Dymax UV-curing System (ECE Series). During UV exposure, an energy of 26.3 J cm -2 was used and the diffraction efficiency was measured before and after UV exposure. The normalized diffraction efficiency was calculated as their ratio. Typical Bragg selectivity curves before and after UV-exposure are presented in Figure . As seen from Figure , the diffraction efficiency of 1b, 1d and BDPI2 decreases to 38, 36 and 40.6%, respectively, after UV exposure. The observed decrease of the refractive index modulation indicates that the bond conversion is the main contributor to the grating formation which assumes that the matrix has low permeability and that the mobile species' diffusion is restricted. Further investigation of the photopolymerization kinetics, relationship between the rate of polymerization and the concentrations of monomer, polymer, dye, and initiator is required in order to develop the model of the grating formation mechanism in CA-PEG sensitized with the studied BODIPY dyes. The model will allow to predict the material response at different special frequencies and achieve its full potential. This will be the focus of our future work.