The mechanism of polyethylenimine-DNA and poly(L-lysine)-DNA complex formation at pH 5.2 and 7.4 was studied by a time-resolved spectroscopic method. The formation of a polyplex core was observed to be complete at approximately N/P = 2, at which point nearly all DNA phosphate groups were bound by polymer amine groups. The data were analyzed further both by an independent binding model and by a cooperative model for multivalent ligand binding to multisubunit substrate. At pH 5.2, the polyplex formation was cooperative at all N/P ratios, whereas for pH 7.4 at N/P < 0.6 the polyplex formation followed independent binding changing to cooperative binding at higher N/Ps.