Compositions for stabilizing polynucleic acids and increasing the ability of polynucleic acids to cross cell membranes and act in the interior of a cell. In one aspect, the invention provides a polynucleotide complex between a polynucleotide and certain polyether block copolymers. The polynucleotide complex can further include a polycationic polymer, as well as suitable targeting molecules and surfactants. The invention also provides a polynucleotide complex between a polynucleotide and a block copolymer comprising a polyether block and a polycation block.