Micelleplexes are promising gene delivery vehicles that form when DNA complexes with polycationic micelles. In this study, the influence of DNA length and topology on the structure and colloidal stability of micelleplexes was explored using a model system. The cationic micelles were composed of poly(2-(dimethylamino)ethyl methacrylate)-blockpoly(n-butyl methacrylate) and were complexed with linear DNA and circular plasmids of 2442, 4708, and 7537 base pairs. The cationic micelles had a mean core radius of 8 +/- 1 nm and a mean hydrodynamic radius of 34 1 nm in buffer at pH 5 and 100 mM ionic strength. The formation of micelleplexes was monitored by turbidimetric titration as a function of N/P ratio (amine in micelle corona/phosphate on DNA) in acetate buffers of various ionic strengths. The structure and size evolution of micelleplexes were studied by dynamic light scattering and cryo-TEM, while the composition of micelleplexes was estimated using static light scattering. The combination of these techniques revealed that increasing DNA length resulted in increased micelleplex size at N/P > 1; this was attributed to an increased propensity for longer DNA to bridge between micelles. At N/P < 1, however, longer DNA enhances the stability of micelleplexes against aggregation by providing additional steric repulsions between micelleplexes. At high ionic strength, increasing DNA length also shifts titration curves to higher N/P ratios as the structure of micelleplexes changes with DNA length. On the other hand, DNA topology showed minimal influence on the titration curves, structure, and long-term stability of micelleplexes. Overall, this work illustrates how polycationic micelles may serve as compaction agents for long chain DNA and how factors such as DNA length can be used to tune the structure and colloidal stability of micelleplexes.