Azo-functionalised materials are of special interest due to their photochromic nature, i.e. reversible trans-cis isomerisation upon photoirradiation. The structure and properties of Langmuir monolayers of nonyloxyphenylazocinnamic acid (NOPACA) with and without exposure to UV-irradiation were studied. The NOPACA floating layer structure was determined by using a quantitative method of compression isotherm analysis. The results showed that the structure of two dimensional face-on M-nanoaggregates, the stable structural elements of floating monolayers, changes when exposed to UV-irradiation. Langmuir-Schaefer films of NOPACA were prepared by deposition of floating layers and studied using X-ray diffraction technique.