We propose to use the new SANS2d to study the dynamic structural evolution in the couse of the formation of DNA-cationic polymer complexes. The dynamic process and the mixing will be supported by using an online stopped-flow system. As this is the first time this type of experiment is tried, a substantial part of effort will be devoted to experimental setup and optimisation of the working conditions. The work will be essnetial for us to identify the contributing roles of structure and charge of the complexes on cell transfection efficiency