The colloidal stability and electrokinetic properties of Stober silica dispersed in acetone-water mixtures containing NaI and CaI2 were investigated. The relative permittivity, epsilon(r), of the dispersion medium was varied between 21 and 79 by controlling the acetone:water ratio. In the presence of NaI, the coagulation concentrations (c(c)) increased from 19.6 to 125 mM as epsilon(r) of the mixtures was raised from 20.7 to 33. However, for epsilon(r) > 33, the particles could not be coagulated using NaI concentrations up to 0.5 M. The silica was sensitive toward Cal(2) additions when epsilon(r) < 24.3 as indicated by the observation of two c(c) values at 0.045 and 2.5 mM when epsilon(r) was 20.7. As epsilon(r) was raised from 24.3 to 78, only relatively small changes in the c(c) were observed from 3 to 7.5 mM CaI2, respectively. The experimental c(c) results could not be predicted by calculations using classic DLVO theory. Zeta potentials (zeta-potentials) for the silica were between -45 and -50 mV throughout the entire epsilon(r) range using 1.5 mM NaI solutions. In the absence of salt, the zeta-potentials remained constant at approximately -50 mV when 24.3 < epsilon(r) < 78.5 but increased to -80 mV for epsilon(r) < 24.3. The zeta-potentials using 1 mM CaI2 solutions remained at about -20 mV from epsilon(r) 78.5 to 33 but changed from -3 to +20 mV as epsilon(r) decreased from 24.3 to 20.7. Theoretical zeta-potentials calculated using the single-site dissociation model were compared with the experimental data. The model predicted that the zeta-potentials should decrease with increasing acetone content (decreasing epsilon(r)). The inability of the theoretical models to predict the colloidal stability of silica in acetone-water mixtures, especially when epsilon(r) > 33, was believed to be due to the presence of a silica gel surface layer which acted as a steric barrier at short-range interparticle distances.