Epoxy-phenolic polymer coatings are widely used for the corrosion protection of carbon steel in the oil production and transportation industries. A primary cause of adhesion loss and coating degradation is the diffusion of aggressive liquids through the polymer matrix. While water sorption by thermosetting polymers has been extensively studied, data on the thermodynamic compatibility of epoxy-phenolic materials with organic solvents remain limited. In this work, the Flory–Huggins interaction parameter (χ) was employed to quantitatively assess compatibility and to predict whether a true solution or a heterogeneous dispersion forms in the polymer–liquid system. Free-standing films with a thickness of 180 ± 25 μm were prepared from an epoxy-phenolic resin cured with three aromatic polyamines: a cardanol-based hardener, diethyltoluenediamine, and methylene-bis(ortho-ethylaniline). Sorption kinetics in distilled water and o-xylene were studied gravimetrically, while changes in thermomechanical properties were evaluated by differential scanning calorimetry (DSC). It was established that the maximum degree of swelling in water decreases in the order: cardanol-based hardener > diethyltoluenediamine > methylene-bis(ortho-ethylaniline), whereas in o-xylene the sequence changes to: cardanol-based hardener > methylene-bis(ortho-ethylaniline) > diethyltoluenediamine. The calculated Flory–Huggins parameters indicate that thermodynamic compatibility is directly governed by the presence of polar groups within the hardener structure. The highest sorption of both water and o-xylene was recorded for the polymer crosslinked with the cardanol-based agent, which is attributed to the presence of hydroxyl groups and a long aliphatic segment. DSC analysis revealed that the glass transition temperature of the polymer based on diethyltoluenediamine did not undergo significant changes following exposure to the aggressive liquid media.

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