Serious pathologies have been linked recently to aluminum toxicity. In this study, model mono- and bilayers were used to get more insight into the interactions of aluminum with phospholipid membranes. To establish if lipid membranes differentiate between cations of a similar electron structure and to determine thermodynamic parameters of Al3+ complexation, both aluminum and praseodymium salts were used. An NMR investigation using phosphatidylcholine unilamellar liposomes in the presence of Al(III) and Pr(III) was performed in order to obtain information on the structure and stability of the complexes formed between the metal cations and the phosphate heads of the lipids. The Langmuir film technique was used to study the influence of AI(III) and Pr(III) on phosphatidylcholine monolayers spread at the air/water interface. Surface pressure and surface potential measurements allowed us to monitor the organization of the lipid molecules and the film property changes. Important differences were observed in the effect induced in the model membranes by the two metal cations. On the other hand, it became obvious that the role of the anion has to be taken into account when rationalizing the interactions of the metal cation with lipid membranes.