The structure of trilayer Langmuir-Blodgett (LB) films on oxidized silicon wafers has been investigated using grazing incidence X-ray diffraction at various incidence angles and atomic force microscopy (AFM). These films are. formed by two behenic acid (BA) layers and a third monolayer of,;amphiphilic molecules having different architectures. These molecules have the same polar head and differ from each other by the chain; either saturated or unsaturated hydrogenated or semi-fluorinated: The structure of the first BA monolayer appears unchanged in all cases, whereas a condensation of the second BA. monolayer. is evidenced. when the third layer. is not formed with the saturated hydrogenated chain. We interpret this condensation as resulting from the mismatch between the lattices of the second BA layer and the external monolayer, possibly associated with the formation of a new monolayer-air interface creating line tension effects. Line tension estimation has also been made from the size of the holes observed in the different LB films.