Polarization modulation infrared reflection spectra of a 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid (DPPA) monolayer on a subphase containing 5 mM tetracycline hydrochloride (TC) were collected under varying surface pressures at the air - water interface. Statistical correlation spectroscopy using 2D IR, βv and kv correlation analyses were performed on these spectra to gain a better understanding of the surface pressure-induced effects on the interaction between the phospholipid and antibiotic. Conventional 2D IR correlation maps revealed strong correlation behavior between the vibrational modes of the lipid and antibiotic. βv correlation plots provided information about the relative rates of occurrence of the coupled responses noted in the conventional 2D IR plots. These calculations indicated that molecular reorientation occurs at lower surface pressures for the modes in Ring A than for the modes in Ring C. A new model-dependent two-dimensional correlation method, exponential kv correlation analysis, confirmed the results from the previous correlation methods and confirmed that the lipid-antibiotic interactions occurred in a bimodal fashion, depending upon surface pressure. The conclusions of the correlation analysis of the surface-pressure induced changes in the DPPA-TC monolayer system lead to the following model for lipid-antibiotic interaction. Initial interaction between the tetracycline molecule and the DPPA molecule occurs at low surface pressures primarily between Ring A of the tetracycline molecule and the lipid headgroup region. However, with increasing surface pressure, the mode of interaction changes, and the strongest interaction at high surface pressures occurs between Ring C of tetracycline and the DPPA headgroup. © 2004 Elsevier B.V. All rights reserved.