Hydrated dipalmitoylphosphatidylcholine (DPPC) multilayers were used as a model membrane system to study the interactions of chlorpromazine hydrochloride (CPZ.HCl) on the lipid phase transition via <sup>31</sup>P NMR spectroscopy. This was studied by measuring the chemical shift anisotropy (CSA) of the <sup>31</sup>P nuclei at 162 MHz using high power proton broadband decoupling. The measurements showed that as the temperature decreases the CSA increases, providing information about the gel and liquid-crystalline phases of DPPC. These data on the pure lipid multilayers were then used as a reference to study the incorporation of CPZ.HCl at different concentrations into the multilayer structure of the lipid. With increasing concentrations of the drug, CPZ.HCl lowered the phase transition temperature of the lipid. The multilamellar structure of DPPC began to break-up at about 10 mol%, by the appearance of a small sharp signal at 0 ppm which increases according to the concentration of CPZ.HCl, and complete break-up was observed at 75 mol%. The additions of increasing CPZ.HCl concentrations resulted in higher values of the CSA of the <sup>31</sup>P nuclei in multilayers.