The electrokinetic potential of barium sulphate, recrystallized from hot concentrated sulphuric acid and subsequently ignited, was investigated against its saturated solution in water by the streaming potential method. The potential is positive and reproducible. The potential of natural barite is also positive but smaller than that of the recrystallized barium sulphate. The difference is not due to impurity in the natural barium sulphate, but it is more likely to be due to irregularity of the surface and consequent distortion of the lattice of the recrystallized barium sulphate. The greater the distortion, the higher is the $\zeta $-potential. The influence of most electrolytes on the potential is completely reversible if there has been contact only at room temperature. Irreversible electrolyte effects are found after prolonged treatment at elevated temperatures. Barium salts and sulphates have a stronger influence on the potential than most other inorganic ions, including multivalent ions such as lanthanum. The potential is a logarithmic function of the activity of the electrolyte solution, but the slope of the curves is smaller than that expected for a thermodynamically reversible electrode. The magnitude of the influence of electrolytes on the potential of recrystallized barium sulphate and of natural barite is very much the same, in spite of the fact that the absolute values of potential of the two types are very different, which is contrary to expectation on the basis of a purely electrostatic theory of ion adsorption. The potential of barium sulphate in its saturated solution cannot be wholly explained by selective adsorption of ions from the solution, but the relative tendencies of the ions to be released from the crystal lattice must be taken into consideration.