Real-time investigation of Bragg angle shift during volume hologram formation is presented in an organic cationic ring-opening polymerization material. Positive as well as negative values of optical shrinkage are found

assumedly related to mechanical deformations of the volume and a change of the average refractive index

respectively. Ruled by the interplay of polymerization and diffusion

the originate grating formation mechanisms prove to represent competing effects regarding the contribution to the optical shrinkage. The influence of sample preparation and holographic exposure procedure on the effects observed is investigated and the usability for minimization of total Bragg resonance detuning is considered.