In this work, racemic proxyphylline ((±)-PXL, a chiral drug) was cocrystallized for the first time with 4-hydroxybenzoic acid (4-HBA) from three solvents (acetone, ethyl acetate, and isobutanol) and using CO2 as an antisolvent. The obtained solid forms were compared to those produced by liquid-assisted grinding (LAG). It is shown that though both methods produce a racemic compound (±)-PXL1:4-HBA1 cocrystal, LAG promotes the crystallization of an orthorhombic Pna21 form (Phase I), whereas CO2-assisted crystallization gives rise to an original monoclinic C2/c form with partial inclusion of CO2 and water molecules (Phase II). The structure of Phase II originates from this inclusion since this phase is never obtained in the absence of CO2. Concomitant cocrystallization of Phase I and Phase II occurs during CO2-assisted crystallization in proportions that vary with solvents and temperature. Our study shows that Phase II transforms progressively into Phase I upon storage or heating.