The application of the Anammox process has been usually focused on the treatment of wastewater with temperatures around 30 degrees C in order to operate under optimum conditions. In this work, the feasibility of the application of the Anammox process at lower temperatures has been tested. First, the short-term effects of temperature on the Anammox biomass were studied using batch tests. An activation energy of 63 kJ mol(-1) was calculated and the maximum activity was found at 35-40 degrees C. Activity tests done at 45 degrees C showed an irreversible loss of the activity due to the biomass lysis. A SBR was operated at different temperatures (from 30 to 15 degrees C) to determine the long-term effects. The system was successfully operated at 18 degrees C but when temperature was decreased to 15 degrees C, nitrite started to accumulate and the system lost its stability. Adaptation of biomass to low temperatures was observed when the specific activities obtained during first batch tests are compared to those obtained during the operation of the SBR.