Experimental cloud-point data are reported for poly(ethylene-co-3.9 mol% methacrylic acid) (EMAA(3.9)) in ethylene, propane, butane, butene, and dimethyl ether (DME) to 250 degrees C and 2500 bar. The cloud-point data for both EMAA(3.9) and poly(ethylene-co-3.9 mol% acrylic acid) are virtually identical suggesting that the structural differences between these two acid copolymers does not have a significant effect on phase behaviour. Cloud-point data are presented that demonstrate the impact of methacrylic acid (MAA) content for EMAA copolymers with 0, 3.1, 3.9, and 5.4 mol% MAA in butane, butene, and DME. The region of miscibility in butane and butene decreases significantly with temperature and with increasing copolymer acid content due to dimerization of the acid groups. However, the region of miscibility in DME increases slightly with MAA content due to the cross association with the acid repeat units. Cloud-point data are reported on the impact of DME and ethanol on the phase behaviour of poly(ethylene-co-3.1 mol% methacrylic acid) (EMAA(3.1)) in butane. At low concentrations, ethanol is a more effective cosolvent since it has two sites that can participate in hydrogen bonding with MAA. The impact of both cosolvents diminishes rapidly with increasing cosolvent concentration once all the acid sites in EMAA are saturated with cosolvent.