A dispersion of soybean lecithin in water leads lo the formation of multilamellar vesicles (MLVs), which on sonication break down into small unilamellar vesicles of approximately 50 nm in diameter, The addition of polymeric molecules in the liposomal system is thought to provide the liposomes with a steric barrier, The molecules used were (tri-) block copolymers (Synperonics) containing a central hydrophobic part (polypropylene oxide) and two hydrophilic chains (polyethylene oxide), The aim of this work was to study whether it was possible to anchor deep inside the lipid bilayer the copolymer hydrophobic block, The exact localization of the copolymer molecules was investigated using a multiprobe technique, The full spectra of two hydrophobic dyes, namely Nile red (NIL) and Pinacyanol chloride (PCYN), were compared while solubilized inside the liposome bilayer, The sensitivity of their spectral characteristics to polarity and self-aggregation produced a monitor of the bilayer microenvironment, The more hydrophobic NIL proved an accurate polarity sensor of the bilayer microenvironment and the formation of PCYN dimers ard nonabsorbing aggregates can be directly related to the local (bilayer) concentration of the dye and the volume available to the solubilized dye molecules, Shifts of the maximum absorbance (lambda(max)) for both dyes showed that the bilayer environment was becoming more apolar with increasing copolymer concentration, The absorbance peak of PCYN due to dimer/aggregate formation increased at moderate copolymer concentrations, indicating that the polymer is incorporated inside the lipid bilayer.