Synthetic gene delivery vectors based on polyelectrolyte complexes of nucleic acids and polycations are widely studied as safe substitutes of viral vectors. Here, we synthesized a series of thermoresponsive graft copolymers (TRC) of poly(L-lysine) (PLL) and poly(N-isopropylacrylamide) (PNIPAM) and evaluated temperature-responsive properties of their polyelectrolyte complexes with plasmid DNA using a range of light scattering techniques. The PNIPAM-containing complexes, swollen bellow the phase transition temperature of PNIPAM grafts, exhibited a significant increase in structural density when the temperature increased above the phase transition. The changes in the structural density of the DNA complexes increased with increasing PNIPAM content and were almost independent of PNIPAM molecular weight. The expected corresponding reduction of sizes of the complexes upon increasing the temperature above the PNIPAM phase transition was observed for complexes of TRC with the highest PNIPAM content and for complexes formed at lower molar mixing ratios. In addition, the surface charge of the complexes was also modulated by temperature. The absolute values of the surface charge (xi potential) increased as the PNIPAM grafts collapsed above their phase transition temperature. The presence of PNIPAM in the complexes resulted in a higher susceptibility toward polyelectrolyte exchange reactions with heparin when compared with parent PLL. In addition, the ability of heparin to liberate DNA from the complexes declined with increasing the time between the formation of the complexes and the addition of heparin. A time-dependent conversion of the supercoiled form of plasmid DNA into the relaxed form when present in the polyelectrolyte complexes was also observed.