Phase-boundary pressures were measured for binary subcritical and supercritical fluid solutions of nonfunctional (referred to as blank), monohydroxy, and dihydroxy polyisobutylenes (200 g/mol) in ethane, carbon dioxide, and chlorodifluoromethane from -50 to 200 degrees C up to 400 bar. As hydroxy groups were added to (blank) polyisobutylene, the phase-boundary pressure was found to increase due to intermolecular self- or cross-association. With carbon dioxide rather than ethane as the solvent, these boundaries are (1) 2-3 times lower in pressure and (2) shift differently than those reported in the literature for 2,5-hexanediol [HO-PE-OH (100)]. To examine these differences, the association parameters for the statistical associating fluid theory model (SAFT) were determined from spectroscopic data and then used to calculate phase boundaries and aggregate size for HO-PIB-OH (200) and HO-PE-OH (100), The SAFT analysis suggests that the steric hindrance due to the methyl branches in PIE decreases the aggregate size and hence lowers the phase-boundary pressure.