Abstract. The physico-chemical methods of determining nitric acid and thorium nitrate in solutions containing these two materials were investigated. Conductimetric titration with sodium hydroxide can be used to determine nitric acid accurately. the titration of thorium nitrate requires a separate standardization of the base however, since a basic salt is precipitated rather the normal hydroxide. The titration of thorium is reproductible however, so and empirical standardization can be used. The measurement of the specific gravity and refractivity of the solutions provides a method of determining the concentrations in terms of these two variable. Equations for the concentrations in terms of specific gravity and refractively are given, both in pure solutions and in those saturated with methyl isobutyl ketone.