Monolayers and LB films of three kinds of carboxylic acid substituted porphyrins without alkyl chain or with alkyl chains of different length have been studied in this paper. Mean molecular areas were measured and found to increase as the alkyl chain length increased. UV-Vis spectra show that soret absorption bands of porphyrins in LB films have different red-shift compared with solution spectra. The polarized UV-Vis spectra show that the three kinds of porphyrins have the same orientation in LB films. The LB monolayers were obtained by using subphase lowing method, their UV-Vis spectra show the same characteristics as those in LB films obtained by using vertical dipping method. These results indicate that substituted alkyl chains have great influence on mean molecular areas and the distance between porphyrin rings in monolayers or LB films, but have no influence on porphyrin ring orientation. The porphyrin ring orientation is determined by porphyrin and the substituted hydrophilic groups. The three kinds of porphyrins have the same orientation in monolayers at Cd^2^+ aqueous solution interface, too, and the orientation preserved during the transfer.

Key words: MOLECULAR STRUCTURE, ORIENTATION, PORPHYRIN, L-B MEMBRANE