This paper aims to show a novel yield stress-zeta potential technique for determining the critical zeta potential of alpha-Al2O3 and ZrO2 dispersions at the flocculated-dispersed state transition and a mean for determining the Hamaker constant of oxides in water via the critical zeta potential. The critical zeta potential is a measure of the particle repulsive potential that exactly counters the van der Waals potential in the flocculated state. At fixed ionic strength the critical zeta potential is indirectly a measure of the strength of the (strongest) van der Waals interaction and hence it can be used to determine the Hamaker constant, an important material property and parameter of the van der Waals equation. Yield stress and zeta potential of dispersions were determined as a function of pH and the subsequent pairing of the yield stress and zeta potential data were made at the same surface chemistry condition. The critical zeta potential was determined from the linear relationship between yield stress and zeta potential square formulated using the DLVO theory. The critical zeta potential was found to be 25 +/- 5 mV for the range of ZrO2 dispersions and 35 +/- 5 mV for alpha-Al2O3 dispersions. This means that the van der Waals attractive force interacting between alpha-Al2O3 particles is twice as strong as that between ZrO2 particles in the flocculated state. Similarly the Hamaker constant of alpha-Al2O3 in water is twice as large.