Photo-induced UV-Vis spectral changes of azobenzene (Az) in solution and aqueous crystalline dispersions were analysed by means predominantly of fourth order derivatives. Weak bands due to vibration level transitions (VLT) of E-isomers in solution were well-resolved in derivatives to estimate conversion of photoisomerisation by tracing extrema of a VLT sub-peak. The photo-induced derivative-spectral changes of Az crystals dispersed in aqueous PVA solutions generated many common crossing points to indicate that Az dissolves partially in a PVA solution to lead to photoisomerisation in a homogeneous phase and the reversible alteration of particle size. The solid-state photoisomerisation of Az was investigated by the preparation of PVA-free aqueous dispersions of powdery nanohybrids comprised of Az and silica nanoparticles. The fourth order derivatives of spectral changes revealed that the photochemical process involves three kinds of species including non-aggregated and aggregated E-isomers as well as Z-isomers.