It is not uncommon to treat plant-derived foods and feeds with alkali. Such exposure to high pH is being used to recover proteins from cereals and legumes, to induce the formation of fiber-forming meat analogue vegetable protein, for preparing peeled fruits and vegetables, and for destroying microorganisms. In addition to their profound effects on functional and nutritional properties in such foods, such treatments may also cause other side reactions, including the destruction of natural polyphenolic compounds. Because plants contain a large number of structurally different antioxidant, anticarcinogenic, and antimicrobial polyphenolic compounds, it is of interest to know whether such compounds are stable to heat and to high pH. In this model study, the stability of the following natural polyphenols to pH in the range 3-11 was studied with the aid of ultraviolet spectroscopy: caffeic acid, (-)-catechin, chlorogenic acid, ferulic acid, gallic acid, (-)-epigallocatechin, rutin, and the nonphenolic compound trans-cinnamic acid. This study demonstrates that caffeic, chlorogenic, and gallic acids are not stable to high pH and that the pH- and time-dependent spectral transformations are not reversible. By contrast, chlorogenic acid is stable to acid pH, to heat, and to storage when added to apple juice. (-)-Catechin, (-)-epigallocatechin, ferulic acid, rutin, and trans-cinnamic acid resisted major pH-induced degradation. The results are rationalized in terms of relative resonance stabilization of phenoxide ions and quinone oxidation intermediates. The possible significance of these findings to food chemistry and microbiology is discussed.