TiO2 and Al2O3 powders were dried at temperatures ranging from 100 to 500-degrees-C and the zeta potential of the solids was measured in various dried organic solvents at room temperature. The inorganic particles were found to be positively charged by an increasing temperature treatment of the solid up to 150-degrees-C, above which temperature the sign of the zeta potential became negative and its magnitude was almost constant in n-alkanes. In order to estimate the position of the shear plane in the electric double layer surrounding the solid surface, the effects of amines (CnH2n+1NH2, n = 1-10), concentration, and alkyl chain length n on the zeta potential of TiO2 particles in n-hexane were investigated. C1-C5 amine adsorption from hexane onto TiO2 particles showed an increase in plateau adsorption with n. Further, the magnitude of zeta potential at the plateaus was found to decrease considerably for longer alkyl chain (n > 4) and this behavior can be explained by a shift of the shear plane toward the solution. Our investigations invalidate the assumption generally made for an organic medium, in which kappa almost-equal-to 0 (kappa is the reciprocal of the Debye length), that there is a very slow decay of the double-layer potential. (C) 1994 Academic Press, Inc.