Nitrification is known as the most temperature-sensitive step among the biological processes in wastewater treatment. The purpose of this study was to investigate the effect of temperature on nitrification, in the case of a sharp decrease of temperature, and to compare this effect with that of a gradual temperature decrease. It was found that a sudden temperature decrease affected nitrification much more than predicted. The immediate decrease of temperature by 10 degrees C led to a 20% larger decrease of specific nitrification rate than predicted by the temperature correction factor of 1.072. The change of nitrification rate resulting from a gradual temperature decrease was modeled correctly with the current default temperature correction factor of 1.072. It was concluded that the correction factor actually can be applied to a gradual temperature-change situation; however, in the case of a sudden temperature decrease, measures need to be taken to avoid nitrifier washout.