Attenuated total reflection (ATR)/Fourier-transform infrared (FT-IR) spectra were measured for one-monolayer Langmuir-Blodgett (LB) films of chlorophyll a (Chi-a) fabricated in an argon (Ar) atmosphere. The frequencies of C=O stretching bands and marker bands for the coordination number of the central Mg atom suggest that Chi-a takes a five-coordinated dimer in the films prepared in the Ar atmosphere. Ultraviolet-visible (UV-vis) as well as IR spectra were obtained for multilayer LB films of Chi-a prepared in air, Ar, nitrogen (N-2), and oxygen (O-2) atmospheres. In the cases of the multilayer LB films, spectral features in the C=O stretching band region suggest that Chi-a exists as a monomer in the film prepared in air while it assumes a dimer in the films prepared in the Ar, N-2, and O-2 atmospheres. The marker bands for the coordination number of the Mg atom in the IR spectra indicate that Chi-a is in a five-coordinated state. These results imply that one can control the molecular arrangement in the LB films of Chi-a by changing the atmosphere in which they are prepared. Both the UV-vis and IR spectra of the LB films fabricated in the Ar, N-2, and O-2 atmospheres are almost identical to each other. This means that O-2 does not affect the stability and structure of Chi-a in the monolayer on the aqueous subphase. On the basis of the postulate that CO2, which exists only in air, caused a change in pH of the aqueous subphase, we investigated pH-dependent IR spectral changes for the LB films. The results indicate that the dimer of Chi-a changes into pheophytine a (Phe-a) below pH 6.0 and that the monomer species do not exist through pH 6.0-4.0; therefore, it is unlikely that CO2 changes the pH. Probably CO2 coordinates to the central Mg atom as a fifth ligand in the LB films prepared in air, preventing the keto carbonyl group of another Chi-a molecule from coordinating to it.