Phase behavior information is necessary for accomplishing homogeneous copolymerization to obtain high yield of copolymers and prevent a fouling problem. Cloud-point data to 160 ℃ and 1,450 bar are presented for five C6 hydrocarbon solvents, normal hexane, 2,2-dimethyl butane, 2,3-dimethyl butane, 2-methyl pentane, and 3-methyl pentane, with poly(ethylene-co-53 mol% norbornene) (PEN53). The pressure-concentration isotherms measured for PEN53/n-hexane have maximums that range between 5 and 12 wt% PEN53. The cloud-point curves for PEN53 all have negative slopes that decrease in pressure with temperatures. The single-phase region of PEN53 in n-hexane is larger than the regions in 2,2-dimethyl butane, 2,3-dimethyl butane, 2-methyl pentane, and 3-methyl pentane. The cloud-point curve of PEN53 in 2,2-dimethyl butane is located at higher temperatures and pressures than the curve in 2,3-dimethyl butane due to the reduced dispersion interactions with and limited access of 2,2-dimethyl butane to the copolymer. Similar cloud-point behavior is observed for PEN53 in 2-methyl pentane and 3-methyl pentane.