Poly(N-isopropylacrylamide) (PNIPAm) co-polymers responsive to temperature and pH were prepared with side chain chemistries in order to exhibit phase transitions under physiologically relevant conditions. Fluorescence spectroscopy, gel retardation assays, dynamic light scattering and atomic force microscopy were used to characterize the binding of plasmid DNA to these materials and to control polymers poly(ethyleneimine) (PEI) and poly(ethyleneimine)-octanamide. Complexes of plasmid DNA with thermoresponsive cationic polymers containing PNIPAm displayed variations in gel retardation behaviour above and below polymer phase transition temperatures, with a high molecular weight linear cationic PNIPAm co-polymer forming complexes with reduced affinity above LCST whereas a branched PEI-PNIPAm co-polymer bound with higher affinity above the PNIPAm phase transition. The thermoresponsive polymers also exhibited changes in particle morphology across the same temperature ranges with polymer DNA complexes prepared at N/P ratios of 2:1 generating spherical particles varying in radius between 30–70 nm at 25 °C and 60–100 nm at 40–45 °C. The transport of DNA within these complexes to cell nuclei was demonstrated to occur within 24 h in tissue culture via confocal microscopy, and low level transfection of mouse muscle cells by a reporter gene encoding green fluorescent protein was achieved with the branched thermoresponsive PEI-PNIPAm conjugate.