Copolymers of 2-dimethyl(aminoethyl) methacrylate (PDMAEM) with N-isopropylacrylamide (NIPAM) were evaluated for their potential to enhance transgene expression of plasmid DNA (pDNA) and gene delivery by adenovirus vectors. The polymers of varying compositions and molecular weights (MW) were synthesized by free-radical polymerization. Polyelectrolyte complexes (PECs) were prepared with different charge (N:P) ratios of PNIPAM/ DMAEM to pDNA. Polymer-modified viral vectors based on non-replicating adenovirus serotype 5 (Ad5), (deltaE1/oriP/luc) or (deltaE1/CMV/luc) transcriptor/promoter/reporter were constructed by electrostatically coupling PNIPAM/DMAEM (Type 1) or PECs (oriP/luc, 6.6 kb) (Type II) to the viral capsid. The N:P value at complete condensation was lower for PECs with higher DMAEM content and MW. pDNA binding was enhanced by high MW PNIPAM/DMAEM. Circular dichroism spectroscopy revealed changes to the secondary structure of pDNA and adenovirus capsid proteins in the presence of PNIPAM/DMAEM. The toxicity of PNIPAM/DMAEM to CNE-1 nasopharyngeal cancer (NPC) cells diminished with decreasing DMAEM content and increasing MW The transfection efficiency ofC666-1 NPC cells by PECs increased with DMAEM content and MW. The transduction efficiency of CNE-1 NPC cells by Type I Ad5 vectors improved with DMAEM content, but was independent of MW. The transduction efficiency of Type II Ad5 in C666-1 cells approximated the sum of expression levels of the PECs and Ad5 vectors individually. PDMAEM and PNIPAM/DMAEM demonstrate both transfection and transduction enhancement activity of modified vectors in nasopharyngeal cancer cells in culture.