The invention provides 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. Preferably, the polynucleotide complex will further include a polycationic polymer. The compositions can further include suitable targeting molecules and surfactants. In another aspect, the invention provides a polynucleotide complex between a polynucleotide and a block copolymer comprising a polyether block and a polycation block. In yet another aspect, the invention provides polynucleotides 10 that have been covalently modified at their 5' or 3' end to attach a polyether polymer segment.