The mutual solubility of TBP and water has been measured in the presence of nitric acid and of various inert diluents for the TBP. Nitric acid partitions have also been studied. It is concluded that the most important compound formed is HNo3.TBP, and that this m a y be partly hydrated. Several new effects occur above ca. 7 M aqueous acidity. Ideal thermodynamic behaviour is observed only at rather low TBP concentrations ; at high TBP concentrations the deviations from ideality are considerable.