Resumen: Biological membranes play an important role in life, protecting and separating the cell from the external environment. Since cell membranes are extremely complex systems is necessary to produce mimic cell membranes in order to study how the different components interact with each other as well as with external agents. In this final master project, the Langmuir-Blodgett technique was used to produce mimic cell membranes using two phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. The aim of this work is to determine the influence of cholesterol in cell membranes and to analyse the thermodynamic aspects of binary (DMPC-cholesterol, DPPC-cholesterol, DMPC-DPPC) and ternary (cholesterol-DMPC-DPPC) mixed systems. The study of phospholipid monolayers in the presence of cholesterol provides information about the cholesterol role in the stability and the membrane organizations, as well as about the lateral molecular packing in the monolayer. The excess molecular area, the compression modulus, excess Gibbs energy of mixing and excess Helmholtz energy of mixing were calculated as a function of the mole fraction proving the condensing effect of cholesterol in phospholipids bilayers.