Densities and water contents of nearly dry and of water-saturated TBP-Amsco 125-82 solutions were measured. From these the molar volumes of TBP and this diluent were calculated to be 273.6 and 246.4 ml, respectively, in the pure dry state. Within experimental accuracy, the molar volume of water was 18 ml in all solutions. Using this value for water, the molar volume of nitric acid in TBP-Amsco 125-82-HNO3-H2O solutions was calculated, from data previously presented (1), to be between 40.9 and 43.7 ml, the specific value depending on the TBP/diluent ratio. Solutions of TBP and the diluent are nearly ideal with respect to additivity of volumes, the maximum deviation from ideality being ∼0.4%, or ∼1 ml per mole of solution based on a diluent gram molecular weight of 185. The water content of TBP saturated with water at 25°C corresponds to a mole ratio .