The solubility of terephthaldialdehyde in N-methyl-2-pyrrolidone (NMP) at temperatures ranging from T = 293.15 to 318.15 K and the mutual solubility for ternary systems of terephthalic acid + terephthaldialdehyde + N,N-dimethylformamide (DMF) and terephthalic acid + terephthaldialdehyde + NMP at T = 298.15, 308.15, and 318.15 K were measured with the technique of isothermal saturation under 101.2 kPa. Six phase diagrams were built on the basis of the determined solubility. There were two solids formed in each ternary system, which were terephthalic acid and terephthaldialdehyde. Each diagram consisted of one cosaturated point, three crystalline areas, and two cosaturated curves. Furthermore, the nonrandom two-liquid mode and Wilson model were used in correlating and calculating the solubility of terephthaldialdehyde in NMP and the mutual solubility data for terephthalic acid + terephthaldialdehyde + DMF/NMP systems. The maximum root mean-square deviation was 3.58 X 10(-3), and maximum value of relative average deviation was 1.74%. The liquid-solid phase diagram and the thermodynamic analysis could provide the basis for obtaining terephthalic acid with high purity.