Cloud-point data to 250 degrees C and 2600 bar are presented for ternary mixtures consisting of either butane or butene with two (co)polymers chosen from polyethylene (PE), poly(ethylene-co-10 mol% methyl acrylate) (EMA(10)), EMA(19), EMA(31), EMA(41), poly(ethylene-co-10 mol% vinyl alcohol) (EVOH10) and poly(ethylene-co-acrylic acid) (EAA(3.9)) The cloud-point curves for ternary mixtures with butane are at higher temperatures than either of the two binary cloud-point curves. The temperatures needed for a single phase increase substantially as the difference in chemical architecture of the two copolymers increases. The cloud-point curves for ternary mixtures with butene are at lower temperatures than the same ternary mixtures with butane. The cloud-point curve for the ternary mixture of 1:2.4 EMA(41):EVOH10 in butane is also at higher temperatures than either of the curves for the two binary mixtures even though the hydroxyl groups in EVOH10] are expected to form hydrogen bonds with the acrylate groups in EMA(41). Small amounts of EAA3.9 cause the PE-butene cloud-point curve to shift by more than 50 degrees C to higher temperature and, likewise, small amounts of PE also cause the EAA(3.9)-butene curve to shift to higher temperatures. Pressure has little effect on the location of the cloud-point curves for all of the systems considered.