The polyelectrolyte complexes formed by DNA and polycations have been widely used as non-viral vectors for gene delivery. The DNA complex prepared in aqueous solutions is usually controlled by kinetics. In this work, we demonstrated, by using salmon testes DNA (2000 bp), poly-(L-lysine) (PLL35), and poly(ethylene oxide)-b-Poly-(L-lysine) (PEO45-b-PLL35), that the DNA complexes formed in dimethylformamide (DMF) was milder than those formed in aqueous solutions. In DMF, no precipitation of DNA complex is observed even at the stoichiometric charge ratio. Taking advantage of the weak complexation and better solubility of complex in DMF, we premixed DNA and PLL at higher concentration in DMF, followed by quenching the mixture into a large amount of aqueous solution. This method alleviates the kinetic control to a large extent, and the complex obtained is smaller in size and lower in molar mass than those prepared solely in aqueous solution. The morphology of the complex, as studied by AFM, is also different. (C) 2013 Elsevier Ltd. All rights reserved.