The phase behavior and mixture densities were obtained for binary mixtures of poly(ethylene-co-20.2mole% butene) copolymer with supercritical n-pentane(n-pentane-h₁₂ and n-pentane-d₁₂). The static type experimental apparatus with a high pressure variable volume cell & linear variable differential transducer(LVDT) was used. The density data were obtained for pure n-pentane-h₁₂ and n-pentane-d₁₂ at the temperature range from 30 to 150 ℃ and the pressure range from 71 to 2415 bar. The pure densities were at the range from 0.530 to 0.750 g/cc and at the range from 0.588 to 0.896 g/cc for n-pentane-h₁₂ and n-pentane-d₁₂, respectively. And also, the phase boundary between single phase/liquid-liquid coexistence region were obtained for 5.3 wt% poly(ethlyene-co-20.2 mol% butene)-n-pentane-h₁₂ system in the pressure range from 10 to 80 bar at 110, 130 and 150 ℃ so thus the equilibrium pressure of phase transition between single phase[L]/liquid-liquid[LL] coexistence region decreased as the temperature decreased. Also, the mixture densities were obtained in the pressure range from 45 to 2415 bar at the same temperatures. The phase transition data between single phase/liquid-liquid coexistence region were obtained for 5.1 wt% poly(ehylene-co-20.2 mol% butene)-n-pentane-d₁₂ mixture in the pressure of below 105 bar at the temperature of 110,130 and 150 ℃. Also the mixture densities were obtained in the pressure up to 2415 bar at the same temperatures. The mixture densities of two systems showed sluggish curve and decreased as the temperature increased.