Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure-area (pi-A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 Angstrom as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface. (C) 2003 Elsevier Inc. All rights reserved.