We have previously used LOQ to identify the complex (polyplex) structures formed between a short single strand DNA and a cationic diblock copolymer MPC30-DEA60. The SANS measurements allowed us to determine the size and shape of the aggregates formed under different charge ratios. We propose to extend this study to the assessment of the effect of the size of DEA using the copolymer series MPC30-DEA30, MPC30-DEA60 and MPC30-DEA100. Deuterated copolymer samples will be used to reveal the more detailed structures of the polyplexes formed. This study will lead to a better understanding of the contributing factors (size and concentration of polyplexes and their charges) on cell transfection efficiency.