Trihalomethanes (THM) concentrations were determined during chlorination of nitrified and partially nitrified secondary effluents, as well as tertiary effluent. Coagulation tests, with alum doses varying from 0.1 mM to 2.5 mM, were applied prior to chlorination to the nitrified effluent in order to remove organic carbon and thereby control the formation of THM. The results indicated that the operational variables of contact time, ammonia nitrogen concentration, pH and chlorine dose had significant effects on THM formation. In tertiary effluents, as the coagulant dose increased, THM formation decreased with a more pronounced reduction for doses higher than 1.5 mM. Modelling of the THM formation was made by means of empirical exponential models. For the chlorination of the nitrified secondary effluent, contact time, chlorine dose, pH and temperature were the model's independent variables, while for the partially nitrified secondary effluent the equivalent independent variables were the contact time, the chlorine dose and the ammonia nitrogen concentration. Modelling of THM formation during chlorination of tertiary effluent was based on either the DOC concentration or the ultraviolet absorbance, as well as on a combination of both. The three exponential models fit reasonably well the experimental data for all the coagulant doses, except for the 2 and 2.5 mM dose, where the reduction in organic matter was not as high as the equivalent reduction in THM formation. The use of the ultraviolet absorbance alone as an independent variable in the model provided a slightly better simulation of the measured THM concentrations.