AWSC (Athens Water Supply and Sewerage Company) uses chlorine as an acceptable disinfectant. An unwanted side effect of chlorination is the formation of by-products. One of the most significant groups of disinfection by-products is the trihalomethanes. Water companies have to keep their concentration levels lower than the limits that the E.U. Directive has set. In search of a relationship between raw water organic carbon, residual chlorine, TTHMs formation and detention time in a water company distribution system, kinetics studies were performed in the laboratory. Chlorine was added to Mornos river raw water and the concentrations of the parameters mentioned above (TTHMs) were determined at regular time intervals by the use of a gas chromatography-electron capture detector (GC-ECD) method. Instead of DOC, absorption in 254 nm wavelength was measured. Two different temperatures were studied. The differences between lab conditions and the real system (AWSC waterworks as well as distribution system) are discussed.