The deformation of giant dioleoylphosphatidylcholine (DOPC) liposomes in solution and the properties of DOPC monolayers were investigated by florescence microscopy and surface pressure-area (π-A) isotherm measurements, respectively. These measurements were taken as functions of pH and ionic strength. When the ionic strength was changed from 0.001 to 0.6 at constant pH of 5.6, the coverage of the DOPC monolayer expanded by 10%, and the liposomes formed small protrusions. When the pH was changed from 5.6 to 3.5 at a constant ionic strength of 0.001, the monolayer coverage shrank by 10%. During this process the external liposome morphology remained the same, but new, smaller vesicles appeared within the interior of the liposomes. Simultaneously changing the pH and the ionic strength to their final values (3.5 and 0.6, respectively), resulted in an expanded monolayer and produced long, protruded liposomes. Our results suggest that the deformation of liposomes is not only driven by osmotic pressure but also the condensed states in each monolayers composing liposome membrane.