The colloidal stability of Stober silica dispersed in acetone (99.7 wt %) was investigated and compared to the results obtained in water using electrophoresis and coagulation experiments with NaI, CaI2, and Bu(4)NI. A critical coagulation concentration (c(c)) of 19.6 mM NaI in acetone was observed whereas Bu(4)NI acetone solutions up to 145 mM had no effect on the stability of the silica organosols. In NaI and Bu(4)NI acetone solutions, the magnitude of the electrophoretic mobilities of the silica organosols decreased with increasing electrolyte concentration. From 0.11 to 13.5 mM NaI the electrophoretic mobilities decreased from -3.3 x 10(-8) to -0.39 x 10(-8) m(2) V-1 s(-1); from 3.9 to 39 mM Bu(4)NI the mobilities decreased from -2.8 x 10(-8) to -0.91 x 10(-8) m(2) V-1 s(-1). Results using a single-site dissociation model were compared with experimental zeta-potentials for silica in NaI acetone solutions and showed adequate agreement when Delta pH was assumed to be 2. Experimental coagulation concentrations for NaI and Bu(4)NI did not agree with simple DLVO theory. The discrepancy was believed to be due to the presence of a steric barrier consisting of a silicic acid gel network. CaI2 additions to the silica acetone solutions influenced the electrostatic potential of the particles and caused coagulation. At 0.06 and 1 mM CaI2, the electrophoretic mobilities of the particles were -2.9 x 10(-8) and +2.3 x 10(-8) m2 V-1 s(-1), respectively. The charge reversal led to the restabilization of the particles as evidenced by two c(c) values 0.045 mM (c(c1)) and 2.75 mM (c(c2)).