The zeta potential data confirmed the adsorption of the large compact Keggin [Si(W12O40)]4- and heteropolytungstate (LST) anions on both α-Al2O3. The adsorbed [Si(W12O40)]4- ions should form a thick steric barrier, ~1 nm, but this effect was not reflected in the yield stress results. A relatively strong additional attractive force explained the increasing maximum yield stress. A similar result was observed with LST. For the spherical α-Al2O3 the largest maximum yield stress (at zero charge) occurred at 0.6 dwb% additive. This attractive force was attributed to patch charge attraction between the negative additive patch and the positive surface site. For the low surface area platelet α-Al2O3 suspensions, complete surface coverage occurred at a low additive concentration as reflected by an almost identical all negative zeta potential-pH behaviour at all additive concentrations at low pH. The slightly larger yield stress at pH 2.5 could be due to this patch charge bridging attraction.