Klorna voda nije, jednostavno, otopina klora u vodi, nego složena otopina koja sadrži elementarni klor, Cl2(aq), klorovodičnu (HCl) i hipoklorastu kiselinu (HOCl) uz trikloridne ione (Cl3−) i možebitno zaostalu kuhinjsku sol, NaCl. Sve te komponente stoje u međusobnoj kemijskoj ravnoteži. U radu je opisana kratka povijest kemije i tehnologije klora, prikazane su ravnotežne reakcije u njegovoj vodenoj otopini te primjena HOCl kao dezinfekcijskog sredstva uz navođenje njezinih kemijskih i fizioloških učinaka. U posljednje je vrijeme razrijeđena klorna voda (50 – 100 ppm) postala popularna kao dezinfekcijsko sredstvo (“elektrokemijski aktivirana voda”, ECA, i sl.). Iako se ECA jednostavno priprema, elektrolizom razrijeđene vodene otopine NaCl, veliku pozornost treba posvetiti njezinoj čistoći te režimu skladištenja kako bi ostala dugo vremena stabilna. Chlorine water is not, simply, chlorine gas dissolved in water, but a complex solution of chlorine, Cl2(aq), hydrochloric (HCl), and hypochlorous acid (HOCl), containing also Cl3− ions as well as NaCl from original electrolyte. All of these components are in mutual equilibria. In this paper a short history of chlorine chemistry and technology is presented, as well as reversible and free-radical reactions in its aqueous solution. Properties of HOCl as a disinfectant and its physiological effects are also discussed. Diluted, 50–100 ppm HOCl solutions, named “electrochemically activated water” (ECA), or similar, have recently become popular disinfectants. Despite the advantage of its simple preparation, by electrolysis of NaCl solutions, ECA is very unstable, so the utmost care must paid to its preparation and protection from light. Diluted chlorine water, if properly prepared and stored, could be stable for months.