The colloidal stability of synthetic silica spheres with clean, methylated, and dehydroxylated surfaces was studied at different concentrations of dissolved gas and KCl electrolyte at a fixed pH of 4.2. A classic stability ratio/electrolyte concentration analysis shows that hydrophobic, methylated particles undergo faster rates of aggregation with increasing concentrations of dissolved carbon dioxide. Similar data for hydrophilic particles and dehydroxylated particles show no change as a function of dissolved carbon dioxide concentration. Zeta-potential data behave similarly, showing a strong influence of dissolved gas only for methylated particles. The results are interpreted in terms of DLVO theory, with the surface-to-surface interaction dominated by the presence of very small, protruding bubbles.