A special spear-tip semisolid electrode has been used to measure the pH of solid oxide-aqueous slurries at very high solid to liquid ratios. A simple non-Nernstian model of the oxide charging along with the experimental results demonstrate a very strong buffering effect of common oxide surfaces at high oxide content. This provides a simple, accurate measurement of oxide PZC which compares favorably to other common methods for PZC determination. The ramifications of the buffering effect on catalyst preparation are discussed.