Adsorption of a charged polyelectrolyte onto an oppositely charged substrate is important for the understanding of many natural molecular events as well as for the development of biomimetic processes for technological applications. In spite of extensive research over the past few decades this seemingly straightforward issue remains poorly understood. One of the experimental constraints is to follow the dynamic structural changes at the interface with sufficient structural resolution. In this work we propose to use the world best neutron reflectometer Inter to determine the structure of cationic PLL layers adsorbed at anionic SiO2 surface and compare the structure and trend of adsorption with theoretical predications.