Pb2+ ion exchange in N-octadecanoyl-L-alanine Langmuir-Blodgett film has been investigated by FTIR spectroscopy. The condensation effect of Pb2+ gives rise to a significant increase in the hydrogen-bonding interaction between adjacent molecules, so that a complete ion exchange occurs after Pb2+ penetrates into the Langmuir-Blodgett film through structure defects. FTIR studies demonstrate that two types of metal complexes are formed with different headgroup coordination and chain orientation: (1) the chelating bidentate complex is formed between pb(2+) ions and carboxylate groups, and the C-C-C planes of the corresponding hydocarbon chains are favorably oriented parallel to the film substrate; (2) the bridging bidentate complex is formed between Pb2+ ions and carboxylate groups, and the C-C-C planes are preferentially oriented perpendicular to the film substrate. The orthogonal orientation of the two types of C-C-C planes is considered to be the probable reason for the monolayer collapse in the presence of Pb2+ and the slow ion exchange within the Langmuir-Blodgett film.