Phase behavior and structural changes were studied for phospholipid monolayers coupled with human serum albumin (HSA) at the air/buffer (pH 3.8) interface by film balance and grazing incidence X-ray diffraction (GIXD) measurements. For comparison, the lipid headgroup was varied from small ionic L-alpha-dipalmitoylphosphatidic acid (DPPA) to large ionic L-alpha-dipalmitoyl-phosphatidyl-L-serine (DPPS) and zwitterionic L-alpha-distearoyl-phosphatidylcholine (DSPC). The presence of HSA leads to a more or less pronounced plateau in the pressure/area isotherms of all phospholipids. GIXD diffraction patterns illustrate that the phase sequences of DPPS and DPPA on buffer (oblique-rectangular-hexagonal) are changed because of the binding of HSA. The oblique-rectangular phase transition disappears and the rectangular-hexagonal phase transition shifts to a lower surface pressure. However, there is no effect of HSA on the structure of the DSPC monolayer (NN tilted rectangular packing). Phase diagrams of DPPA/HSA, DPPS/HSA, and DSPC/HSA have been established. A tilt angle decrease in both DPPA/HSA and DPPS/HSA monolayers is observed, but no change of the tilt angle for DSPC molecules in the mixed DSPC/HSA monolayer is found. Thus, whereas HSA binding stabilizes the liquid-expanded phase at low pressures most probably via penetration, it condenses the ordered phases of anionic monolayers via reducing the headgroup repulsion. The positional correlation lengths xi(i) parallel and perpendicular to the tilt direction change because of the binding of HSA.