The present invention relates generally to the amphiphilic polyelectrolyte, poly(2-ethylacrylic acid) and covalently bonded lipids to generate Lipo-PEAA. These Lipo-PEAA are then used to make pH-sensitive liposomes which become unstable, permeable or fusogenic with certain pH changes. In addition, this invention generally describes methods for delivering therapeutic compounds and drugs to target cells by administering to a host the pH-sensitive liposomes of the present invention.