Polarized Fourier transform infrared spectra of black lipid films in air withdrawn from aqueous dispersions of dimyristoylphosphatidylcholine (DMPC) have been recorded at various electrolyte concentrations. The spectra revealed that monovalent cations gave little effect, whereas polyvalent cations except Mg2+ gave remarkable effects on vibrational wavenumbers of both the acyl chain and phosphate ester group of DMPC in black lipid films. In the case of Mg2+, the effect was discerned only for the phosphate ester group. By use of thin film optics, molecular orientation of DMPC in the black lipid film in air was quantitatively evaluated as a function of electrolyte concentration. In the case of monovalent and Mg2+ cations, the orientation angle of the acyl chain axes from the surface normal was little affected by the electrolyte addition. On the other hand, the other polyvalent cations caused decrease of the average orientation angle from ca. 40 degrees to 30 degrees. The affinity of the cations for the DMPC lipid films in air are in the order of Zn2+ > La3+ similar to Ca2+ similar to Pb2+ > Mg2+ > Na+ similar to Li+.