A study of the extraction characteristics of nitric acid with octyl(phenyl)-N,N-diisobutylcarbcrmoylmethylphosphine oxide [O phi D(iB)CMPO or, simply, CMPO] was conducted. In the experimental program, CMPO was dissolved in n-dodecane to produce the organic extracting medium. The objectives of the project were to infer extraction stoichiometry and to estimate equilibrium constants for the extraction of nitric acid with the CMPO extractant. Experiments were performed over a limited range of concentrations to avoid conditions favoring formation of a third phase. Aqueous nitric acid concentrations were limited to 0.30 M at 25 degrees C, 1.0 M at 40 degrees C, and 3.0 M at 50 degrees C. The data indicate that CMPO extracts nitric acid with a 1:1 stoichiometry. The value of the equilibrium constant is estimated at 2.66 +/- 0.09 at 25 degrees C. The enthalpy of the extraction is estimated to be - 5.46 +/- 0.46 kcal/mol.