Cloud-point data to 260 degrees C and 2600 bar are presented for four different solvents, propane, butane, butene, and dimethyl ether (DME), with polyethylene (PE) and four poly(ethylene-co-acrylic acid) copolymers (2.4, 3.9, 6.9, and 9.2 mol % acrylic acid). The pressure-concentration isotherms measured for PE-DME and poly(ethylene-co-3,9 mol % acrylic acid)-butene mixtures have broad maximums that range between 5 and 10 wt %. The cloud-point curves for the acid copolymers in the olefinic and paraffinic hydrocarbons dramatically increase in pressure to as high as 2600 bar with decreasing temperature, especially below similar to 180 degrees C where intra- and interpolymer acid dimerization occurs. All of the cloud-point curves in the olefinic solvents are located at lower temperatures and pressures than the curves in the analog paraffinic solvents. The cloud-point curves for the acid copolymers in DME are all below 600 bar as a result of the hydrogen bonding between the acrylic acid units and DME. The PE cloud-point curves for the paraffins and olefins all have positive slopes and are located at pressures below 800 bar. In contrast, the PE-DME curve has a negative slope that increases rapidly in pressure at temperatures below 140 degrees C where polar DME-DME interactions dominate the phase behavior. Cloud-point curves in butene are affected more by the acid content of the copolymer than by molecular weight. At pressures greater than 1000 bar, the curves shift to higher temperatures by approximately 26 degrees C/mol % acid compared with 1.7 degrees C/10 000 M(n), up to an M(n) of 132 000. Also, the curves isobarically shift to higher temperatures with increasing M(n) rather than M(w), indicating that the phase behavior may be more sensitive to the number rather than the weight of acid groups in the backbone.