In the present study, mixed liposomes of dihexadecyl phosphate sodium salt:phosphatidylcholine:cholesterol at a 1:19:9.5 molar ratio were allowed to interact with poly-L-arginine at temperatures below and above the main phase transition of the liposomal membrane. The interaction led to the formation of aggregates, which gradually increased in size and eventually precipitated. It was, however, possible, during the initial stage of the experiments, when the ratio of guanidinium group relative to phosphate was smaller than ca. 40%, to determine their size and charge and observe their morphology in aqueous dispersion. Fluorescence experiments established that the liposomes are not ruptured during their interaction with poly-L-arginine. Instead, they are attached at the polypeptide chain through the guanidinium-phosphate complementary pair. Fluorescence quenching experiments indicated that the poly-L-arginine chain is accessible for interaction with iodides dissolved in the aqueous phase when the temperature of the liposomal dispersion is below the main lipid phase transition. It is, however, partitioned in the interior of the membrane at temperatures exceeding this main lipid phase transition.