The mutual solubility of terephthalic acid + p-toluic acid + N-methyl-2-pyrrolidone (NMP), terephthalic acid + 4-carboxybenzaldehyde + NMP and p-toluic acid + 4-carboxybenzaldehyde + NMP systems at T = 353.15, 323.15, and 298.15 K were experimentally determined by means of a static technique under local atmospheric pressure (P = 101.2 kPa). Nine isothermal phase diagrams were constructed in terms of the solubility data determined. Two neat solids were formed in each isothermal system, which were adduct 1:2 terephthalic acid-NMP (298.15 and 323.15 K) or terephthalic acid (353.15 K) and p-toluic acid for the ternary terephthalic acid + p-toluic acid + NMP system, 4-carboxybenzaldehyde and p-toluic acid for the ternary 4-carboxybenzaldehyde + p-toluic acid + NMP system, and adduct 1:2 terephthalic acid-NMP (298.15 and 323.15 K) or terephthalic acid (353.15 K) and 4-carboxybenzaldehyde for the terephthalic acid + 4-carboxybenzaldehyde + NMP system. The adduct terephthalic acid-NMP (mole ratio, 1:2) appeared at below 323.15 K when the terephthalic acid dissolved in NMP. Each diagram consisted of two co-saturated curves, one co-saturated point, and three crystalline areas. The phase diagram determined in this contribution could provide the basis for separating the mixtures of p-toluic acid, 4-carboxybenzaldehyde, and terephthalic acid.