4-Chloroaniline (4-ClA) is a chlorinated aromatic amine that is formed as an intermediate during the microbial decomposition of phenylurea and phenylcarbamate. The formation of various oligomers by polymerization of 4-ClA with guaiacol in an aqueous solution containing oxidoreductases has been reported. 4-Chloroaniline is used as an intermediate in the production of a number of products, including agricultural chemicals, azo dyes and pigments, cosmetics, and pharmaceutical products. Reactive metabolites of 4-Chloroaniline bind covalently to haemoglobin and to proteins of liver and kidney. In humans, haemoglobin adducts are detectable as early as 30 min after accidental exposure, with a maximum level at 3 h. Repeated exposure to 4-Chloroaniline leads to cyanosis and methaemoglobinaemia, followed by effects in blood, liver, spleen, and kidneys, manifested as changes in haematological parameters, splenomegaly, and moderate to heavy haemosiderosis in spleen, liver, and kidney, partially accompanied by extramedullary haematopoiesis. The marketing and use of products containing 4-Chloroaniline based azo dyes were banned by the European Union (EU) (EC, 2000).