Hierarchically structured materials, found in nature in various forms, often exhibit a complex profile of properties even though they only consist of a rather small spectrum of chemical elements. The variations in their macroscopic properties can be attributed to the structure of the materials and therefore various measuring techniques are required for the characterization of the structure on all levels to be able to exploit the materials’ full potential. In this work composites made from cellulose nanocrystals (CNC) and polyethylene glycol (PEG) were synthesized and characterized using UV-vis spectroscopy as well as small-angle x-ray scattering. The main task of this work was the characterization of the photonic structure of the CNC/PEG composites depending on the polyethylene glycol content in order to gain a better understanding of the optical properties of these films. For this purpose, CNC/PEG composite films with different PEG contents ranging from 0 to 40 weight percent were synthesized and characterized with small-angle x-ray scattering at several angles. It was shown that with increasing amount of PEG the layer spacing of the cholesteric structure increased resulting in a shift of the reflected colour of the films towards red. Further, small-angle x-ray scattering experiments were performed at elevated relative humidity levels to get a deeper insight into the reversible change of the chiral nematic structure of the CNC/PEG composite films on the nanoscale when subjected to humidity. The subsequent evaluation of the SAXS data in the form of a two-dimensional indirect Fourier transformation (IFT) yielded one-dimensional as well as two-dimensional real space functions out of which structural parameters of the CNC/PEG composite films could be qualitatively extracted.