Insertion properties of Lasalocid and Monensin as sodium salts (LAS-Na, MON-Na) in model membranes of dipalmitoylphosphatidylcholine (DPPC) have been studied in two biomimetic buffered systems: monolayers (Langmuir Film Balance) and multilamellar bilayers (Differential Scanning Calorimetry). Effects varying with the concentration and with the surface pressure have been observed within the experimental range of molar ratios r = antibiotic/lipid from 0.005 to 0.1. The area occupied by each antibiotic molecule and the relative molecular area increase at a given pressure, are different for the two molecules. The calorimetric measurements result in concentration dependent modifications of the characteristics of the gel-liquid crystal phase transition in the case of LAS-Na. No such modifications are observed in the case of MON-Na.