The influence of the addition of several inert compounds on the phase equilibria of poly(ethylene-co-l-hexene) copolymer solutions in l-hexene and ethylene was determined at the conditions of the industrial high-pressure copolymerization process. Using a high-pressure autoclave we measured cloud point pressures in dependence on temperature and mixture composition. The temperature was varied in the range of 393 to 493 K. We present the results for quasi-ternary mixtures consisting of a poly(ethylene-co-1-hexene) copolymer (EH), ethylene and an inert compound (helium, nitrogen, CO2 methane, ethane, propane, n-butane). Also the quasi-quaternary system EH-copolpmer/ethylene/1-hexene/nitrogen is discussed. Experiments were carried out at 1-hexene:ethylene mass ratios between 0 and 85:15. Helium, nitrogen, methane and carbon dioxide lowered the copolymer solubility. The antisolvent effect decreases in the order helium > nitrogen > methane > CO2. In contrast, ethane, propane and especially n-butane were found to act as cosolvents.