A triple-chain phospholipid monolayer at the air-water interface is in
vestigated by means of grazing incidence X-ray diffraction (GID). Anal
ysis of the diffraction spot profiles parallel and perpendicular to th
e surface yields different ordered phases. On increasing the lateral p
ressure at different temperatures the hydrocarbon chains form tilted p
hases with a tilt toward the nearest neighbors and eventually a hexago
nal lattice with vertical chain orientation.