We have studied the interactions between an active ingredient (SR 33557) and polyethylene glycol 6000, an excipient. The phase diagram has been established for the compositions, ranging from 3 to 97% w/w PEG 6000. The eutectic mixture was found at 80% w/w PEG 6000; its fusion temperature and its enthalpy were T-E = 59.3 +/- 0.3 degrees C, and (Delta H-fE)(exp) = 176.0 +/- 0.5 J g(-1), respectively. We have also tried to describe and quantify the interactions which characterise the eutectic by determining the enthalpy, the entropy, the "free" mixture enthalpy and the Flory-Huggins parameter of interaction "chi" of two chemical compounds. These interactions would partly explain why the solubility of the active ingredient (SR 33557) is improved when it is associated with a polymer, for the eutectic mixture, and why such diagrams are necessary for pharmaceutical application.