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A series of novel water-soluble, bioreducible poly(amido amine)s (SS-PAAs) was synthesized by Michael addition polymerization of N,N′-dimethylcystamine (DMC) with various bisacrylamides. DMC-based SS-PAAs show high buffer capacities in the pH range 5.1–7.4, facilitating endosomal escape. The polymers form nanoscaled (<250 nm) and positively charged (>+20 mV) polyplexes which are relatively stable, but rapidly disintegrate in the presence of 2.5 mM DTT. These polyplexes have transfection efficiencies up to 4 times higher than bPEI in COS-7 cells, with low cytotoxicity.