The molecular organization of purely aromatic, polyphenyl carboxylic acids, as Langmuir monolayers at the air/water interface, has been investigated by means of surface pressure and electric surface potential measurements upon film compression. The monolayer characteristics of the basic compound, a symmetrical triphenylbenzene (5 ' -phenyl-m-terphenyl) ring with a carboxylic group at the 4 position (namely 5 ' -phenyl-1,1 ': 3 ' ,1 " -terphenyl-4carboxylic acid), are compared with those of its derivatives containing hydrophilic (nitro) or hydrophobic (phenyl) substituents. The nature of the substituent as well as its position (2 ' or 4 ') has a profound influence on the monolayer properties. The results are discussed in view of molecular orientation deduced from values of effective dipole moments.