The synthesis and properties of pH-sensitive polyethylene glycol (PEG) lipids are described. The sensitivity of these conjugates to slightly acidic pH was clearly related to the structure of the orthoester linkage involved. It was found that pH-sensitive PEG lipids stabilized cationic lipid/DNA isoelectric complexes as efficiently as their non-pH-sensitive PEG analogs at neutral pH. Lowering the pH resulted in the precipitation of the complexes bearing pH-sensitive PEG lipids as a consequence of their degradation. In contrast, insertion of non-pH-sensitive PEG lipids maintained the complex colloidal stability even at lower pH. In vitro results showed a significant increase in transfection with formulations containing pH-sensitive PEG lipids versus non-pH-sensitive analogs. These conjugates show promising properties as lipoplex-stabilizing agents at neutral pH, which could be triggered by a mild acidic environment such as that occurring in solid tumors, inflammatory tissues, and intracellular endosomal compartments.