Our objective was to determine if the acidity of whey protein solutions was responsible for their astringency. Panelists rated acidic whey protein and acid-only solutions for astringency and sourness. Acidic whey protein solutions contained 6% or 1% whey protein isolate and phosphoric acid at a pH of 3.4. Acid-only solutions were formulated to match the whey protein solutions for either total acidity or for pH. The acid-only solutions matched for total acidity were more astringent than the whey-containing solutions, while those matched for pH were significantly less astringent. Sourness was reduced by the whey proteins, most likely because of the decreased concentration of free hydrogen ions. The astringency of acidic whey protein solutions appears to be caused by their high acidity and not directly by the whey proteins.