Peroxynitrite can oxidise and nitrosylate biomolecules and is associated with several diseases. The peroxynitrite scavenging of substituted phenols and several flavonoids was studied. The activity of phenol (poor scavenger) is positively influenced by electron donating substituents. A good correlation was found between the peroxynitrite scavenging activity of the substituted phenols and the Hammett sigma or the E-HOMO. Flavonols containing a catechol group (3'- and 4'-OH) in ring B (rutin and monohydroxyethyl rutoside) or an AC-ring with three OH groups (3-, 5- and 7-OH) were potent scavengers. Evidence has been produced that in the AC-ring the 3-OH group was the reactive centre and that the reactivity of this group was positively influenced by electron donating groups at the 5 and/or 7 position (galangin, kaempferol, trihydroxyethyl quercetin).