The influence of PEG in the physicochemical stability of liposomes has been determined. Vesicles composed of PC/PG/Ch/DX (A), PC/PG/Ch/DX/PE-PEG (B), PC/PG/Ch/DX/PE-PEG/NGPE/E(8) (C), and PC/PG/Ch/PE-PEG/NGPE/E(8) (D) were prepared and characterized. The stability of these four liposomal preparations was studied via its ability to spread on air/water interfaces and the microviscosity of its bilayers. Preparations lacking PEG show no tendency to spread on aqueous surfaces, thus indicating a high stability of these types of preparations. Only if liposomes were previously treated with methanal can one observe, as expected, surface activity. On the contrary, liposomes containing 5% of PEG are able to form monomolecular films both after spreading on aqueous surfaces and upon injection into the bulk of a water solution. The contribution of entrapped drug and coating peptide is not important in the range of concentrations under study. Microviscosity studies carried out through fluorescence polarization changes of DPH with temperature shaw that all the samples are very stable (high polarization values) in the range of temperatures under study. Nevertheless, the presence of PEG has, in general, a slightly fluidifying effect. There is a certain interaction between Doxorubicin and DPH in the sense that this drug reduces the fluorescence intensity of the marker, but it has no influence on the anisotropy values, thus allowing us to carry out the study without interferences or artifacts.