THM is a group of substances that are a byproduct of the chlorination of drinking water. In the presence of organic substances in water and by chlorination, obtain THM, as some of the most common are: chloroform, dihlorbrommetan, dibromhlormetan, bromoform. Besides being an indicator of organic pollution of drinking water, they are probablycarcinogenic. Besides these, there are listed series of other organochlorinesubstances in lower concentrations, so THM determine by the first four.In preparing their analysis, there are serious problems because they are highly volatile. They are analyzed by gas chromatograph equipped with electron capture detector.