The solubility of thorium was studied in 0.1 M NaCl solution under anoxic carbonate-free conditions. The steady state was approached from both oversaturation and undersaturation. The equilibration periods used ranged up to 71 days. The crystallization of the formed amorphous or microcrystalline solid Th phases was examined by X-ray diffraction. With ICP-MS, the measured Th concentrations in solution were between 10-11 and 10-8 M. No indication of Th colloids was detected in the SEM and TEM examinations of the ultrafiltration membrane specimens and the aliquots of solution prepared before and after ultrafiltration. The measured Th concentrations agreed fairly well with the computed values (EQ3/6 code) of Th concentration, about 2 × 10-9 M under CO2-free conditions.