Zeta potential (ζ)-pH and yield stress (τ_y)-pH behaviour of a number of kaolinite and montmorillonite slurries, including CMS-sourced materials, were compared. The CMS KGa-2 and Crown kaolin with very similar elemental composition displayed almost identical ζ-pH and τ_y-pH behaviour. Both displayed a pH_ζ=0 at 3–4 where the maximum τ_y was located. This pH_ζ=0 was higher at higher ionic strength as the pH-dependent charge increased with ionic strength and the permanent structural charge being invariant. The other kaolinite slurries (Reidel and Unimin) with different composition showed different behaviour. The surface chemistry and rheological properties of CMS-sourced SWy-2 (Na-) and STx-1b (Ca-) montmorillonite and bentonite slurries were also compared. They all displayed a negative zeta potential that was insensitive to pH. STx-1b slurries required a much higher solid concentration to form a gel. Maximum τ_y occurred over a broad range of pH. This pH is ∼5 for SWy-2, 8 for STx-1b, <2 for bentonite and 12 for API bentonite. Their difference in clay mineral composition such as impurities and exchangeable cations were highlighted. The point of zero charge (pzc) of kaolinite and montmorillonite slurries obtained via Mular-Roberts pH-salt addition method did not correlate well with the pH_ζ=0 except for the KGa-2 and Crown kaolin.