The present invention provides ortho ester lipids and their derivatives that, upon certain pH conditions, undergo hydrolysis with concomitant or subsequent headgroup cleavage. These ortho ester lipids can advantageously be formulated into liposomes. The liposome formulations are useful in nucleic acid transfection and entrapment/delivery of conventional small molecules and therapeutic agents. Moreover, the liposomes comprising compounds of the present invention are useful as drug delivery vehicles.