Amphotericin B (AmB) monolayers exhibit a phase transition at a surface pressure of 10 mN/m, indicated by a plateau in the T-A curves, which is attributed to the reorientation of the AmB molecules from a horizontal to a vertical position. In mixed films with dipalmitoyl phosphatidyl serine (DPPS), this plateau appears at a higher surface pressure, 20 mN/m, independently of the mixed monolayer composition. It is suggested that the presence of the phospholipid in the mixture hinders the reorientation of AmB molecules as a consequence of the formation of a 2:1 AmB/DPPS complex. With the aid of phase diagrams and by application of the Crisp phase rule, the miscibility of the mixed films' components is discussed. To study the monolayers' behavior in the collapse region, subphases of high ionic strength (3 M aq. NaCl) were used. Under these conditions, the existence of two independent collapses was observed; the first corresponds to the ejection of the AmB molecules from the surface, and another is related to the expulsion of the phospholipid. Both components were separated from the 2:1 AmB/DPPS complexes when the monolayer was compressed above 20 mN/m.