We propose a calorimetric study of phospholipid model membranes (DPPC) interacting with stearylamine (SA) and a short polypeptide. A pentapeptide, thymopentin (TP5), whose sequence is Arg-Lys-Asp-Val-Tyr, was studied and the following aspects were examined : (a) effect of increasing amount of SA on DPPC liposomes, (b) effect of TP5 on pure DPPC liposomes at different pH, and, (c) effect of TP5 on model mixture of DPPC-SA at different pH. Results show that pH plays a fundamental role in modulating TP5 interaction with zwitterionic DPPC membranes. This interaction becomes more complex in the presence of high SA concentration in the membrane, because of the opposite charges gained by SA and TP5 at acidic or alkaline pH. The pH-induced charges cause either a larger stabilization or fluidification of the DPPC-SA bilayer.