The Hildebrand solubility parameter (δ), which predicts the miscibility of materials, was determined for the compounds that are involved in the oxidation of p-xylene to terephthalic acid. The δ values were determined from the measured intrinsic viscosity [η] of the compounds. The following compounds (namely, methyl benzoate, p-tolualdehyde, 4-hydroxymethyltoluene, terephthalaldehyde, benzoic acid, p-toluic acid, 4-hydroxymethylbenzoic acid, phthalic acid, trimellitic acid, 4-carboxybenzaldehyde, and isophthalic acid, including p-xylene as the reactant and terephthalic acid as the main product) were studied. The predicted δ values were obtained using the group contribution method, which helped in selecting the solvents to be used for viscosity measurements. Mixtures of solvents were used to obtain δ values for phthalic acid, isophthalic acid, terephthalic acid, and trimellitic acid. It was observed that δ is apparently related to the melting points of the compounds. Three-dimensional Hansen solubility parameters were also determined for important p-xylene oxidation products for a better understanding of their solubility behavior.