The solubility of a commercial aryl phosphite processing stabilizer for polyolefins, {tris (2,4-di-t-butylphenyl)-phosphite) (Irgafos 168, Ciba Specialty Chemicals), has been measured in polypropylene over a range of temperatures. Measurements are complicated by hydrolysis of the phosphite, releasing 2,4-di-t-butylphenol, which is much more soluble in the polymer. We have measured the solubility of this phenol, and of the phosphate oxidation product of the stabilizer. The results are interpreted on the basis of a regular solution model. They show that the phosphite has a very low solubility but that it is not particularly anomalous in comparison with other stabilizers of similar molecular weight and polarity. The phosphate is significantly more soluble than the phosphite at 70 degreesC, but their solubilities are comparable at 25 degreesC. This is mainly due to the lower heat of fusion and melting point of the phosphate, whose compatibility is actually lower than that of the phosphite. The phenol is much more soluble because of its low melting point.