The performance of an upflow packed bed biofilm reactor has been analyzed under multi-substrate limitation by considering simultaneous carbon oxidation and nitrification reactions. For an inlet concentration of 200 mg l(-1) of NH4+-N and 50 mg l(-1) of methanol, the results show that organic substrate is consumed very rapidly near the inlet of the reactor while most of the NH4+-N is converted in the central region of the reactor. When the inlet concentration of methanol is subjected to sinusoidal variation, the exit concentration of NH4+-N also follows a sinusoidal pattern although its inlet concentration was subjected to step input. However, the methanol exit concentration exhibits the typical step input response when the inlet NH4+-N concentrations subjected to sinusoidal variation. The concentration profiles within the biofilm reactor show that oxygen is a limiting component at the middle of the reactor when both methanol and NH4+-N inlet concentrations follow sinusoidal variation.