A ternary oxide of nominal composition Ru0.3Ti0.3Ce0.4O2, prepared by thermal decomposition at 550 degrees C of the chlorides, was used to determine charge-pa curves in KNO3 aqueous solutions at seven concentrations of the electrolyte. The purpose was to discuss the Parsons-Zobel plot (1/C vs. 1/C-d, where C is the capacitance at the point of zero charge) in terms of the spherical vs. planar geometry of the diffuse layer to calculate C-d by means of the Gouy-Chapman theory. The results indicate that the planar geometry of the double layer is more appropriate, although only up to comparability of the Debye length with the particle size.