The apparent electrokinetic potential ζa of tin sulphide sol has been measured both by electro-osmotic and electrophoretic methods in the region of slow coagulation using Smoluchowski's equation in the presence of equicoagulating concentrations of electrolytes containing counter-ions of different valency. From the apparent values of ζa the true ζ was found by applying surface conductance correction using the equation 1/ζa=1/ζa+m·1/S where m is a constant involving specific surface conductance and S the specific conductance in bulk of the solution in contact with the colloid particles. The results lead to the conclusion that a given rate of coagulation of the tin sulphide sol by different electrolytes is characterised by a definite value of the true ζ-potential, independent of the valency of the counter-ions used. The average value of true ζ-potential of the particles of the sol determined by the electrophoretic and electro-osmotic methods is equal to 29.6 m.v. when the rate of coagulations is slow (20 hours). © 1958 Dr. Dietrich Steinkopff Verlag.