The solubilities of bis(4-methylphenyl)phosphine sulfide (BMPPS) and bis(4-carboxyphenyl)phenylphosphine oxide (BCPPO) in acetic acid were measured. The concentration of the solution was determined by sodium hydroxide with phenolphthalein as indicator. The solubilities of BMPPS were measured over the range of 64-104 degreesC, and the solubilities of BCPPO were measured over the range of 66-106 degreesC with uncertainties of 3% and 2%, respectively.