The solubilities of 2-carboxyethylphenylphosphinic acid (CEPPA) and a potential reactive flame retardant, 4-carboxyphenylphenylphosphinic acid (CPPPA), in water were measured. The concentration of the solution was determined by sodium hydroxide titration with phenolphthalein as the indicator. The solubilities of CEPPA were measured from 25.10 degreesC to 76.32 degreesC with an uncertainty of 2.2%, and the solubilities of CPPPA were measured from 26.70 degreesC to 94.52 degreesC with an uncertainty of 2.1%.