Phase transitions both in mono- and multilayers have been studied for homologous methyl-branched chain 1-ocyl-2-alkyl-phosphatidylethanolamines (PE) differing in acyl chain lengths. In bulk systems two phase transitions have been observed. While with increasing chain length the main transition temperature increases, the second transition observed at higher temperatures shifts to lower values. In monolayers at the air-water interface both T-0 and T-c increase with increasing chain length. These characteristic temperatures were obtained from the linear extrapolation of transition pressure (pi(c)) and area involved in the transition as function of temperature towards zero values, respectively. The structures of condensed monolayers have been characterized using grazing incidence X-ray diffraction (GIXD) measurements. The introduction of a methyl-branching in double chain PEs drastically increases the tilt angles of the hydrocarbon chains. The transition towards a hexagonal untilted phase is completely suppressed. Compared with the symmetrical non-branched dipalmitoylglycerophosphoethanolamine (DPPE), 1(2C(1)-18:0)-2H-PE shows a change in the tilt direction towards next nearest neighbor (NNN) and 1(2C(1)-15:0)-2H-PE displays a superposition of multiple lattices of constant tilt but different tilt azimuths,