Cell membranes often constitute the first biological structure encountered by drugs, and binding or interactions of drugs with lipid components of the membrane may explain part of their mechanism of activity or their side effects. The present study provides evidence of alterations in the structural properties of phospholipid bilayers at acidic conditions that can be correlated with the mechanism of action of nonsteroidal anti-inflammatory drugs (NSAIDs) and with their local action effect on the gastrointestinal tract lipids, aiming a molecular biophysical approach to the interaction of these drugs with lipid membranes. In this context, the structural modifications of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers at pH 5.0, induced by increasing concentrations of five NSAIDs (piroxicam, meloxicam, tolmetin, indomethacin, and nimesulide), were studied by small-angle and wide-angle X-ray scattering. Results obtained highlight the effect of each NSAID in modulating the membrane structure properties. All the NSAIDs promoted distinct biophysical effects by perturbing the membrane arrangement to different degrees that are intimately related to their different physicochemical properties as well as with the initial organization of the lipids, depending if they are in the gel (L(β')) or in the liquid-crystalline phase (L(α)).