The Hildebrand solubility parameter (delta), which predicts the miscibility of materials, was determined for the compounds that are involved in the oxidation of p-xylene to terephthalic acid. The delta values were determined from the measured intrinsic viscosity [eta] 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 delta 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 delta values for phthalic acid, isophthalic acid, terephthalic acid, and trimellitic acid. It was observed that delta 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.