The solubility (from 0.95 degrees C to 19.11 degrees C) and metastable zone width of phosphoric acid in tributyl phosphate (TBP) have been measured using a laser method. The results show that the solubility curve is remarkably dependent on temperature in the experimental temperature range. A polynomial fitting of the solubility data was obtained. The metastable zone width increases directly with the increasing rate of cooling but decreases with the increasing concentration of phosphoric acid in TBP. An apparent secondary nucleation order m for phosphoric acid in the TBP system was calculated using Nyvlts approach and revised using a modified linear regression method. The effects of typical impurity ions (Al3+, Mg2+, and Fe3+) on the metastable zone width for phosphoric acid in TBP were also investigated. The presence of impurity ions can enhance the metastable zone width for phosphoric acid in TBP.