Phenolic acids or phenolcarboxylic acids are types of aromatic acid compound. Included in that class are substances containing a phenolic ring and an organic carboxylic acid function (C6-C1 skeleton). There are several categories of phenolic acids including:

• Monohydroxybenzoic acids: salicylic acid, 3-Hydroxybenzoic acid, 4-Hydroxybenzoic acid
  • Some esters of this group include paraben, methyl paraben, propyl paraben
• Dihydroxybenzoic acids: vanillin, vanillic acid, gentisic acid, protocatechuic acid
• Trihydroxybenzoic acids: gallic acid, ellagic acid, syringic acid, eudesmic acid, phloroglucinol carboxylic acid
  • This type of phenolic acids (especially gallic acid, ellagic acid) is a component of hydrolysable tannins.