The performance of an upflow packed-bed biofilm reactor (UPBR) was analysed under multi-substrate limitation by considering simultaneous carbon oxidation and nitrification reactions. Numerical solutions of the proposed model defining the system were obtained for a wide range of operating conditions to illustrate individual substrate limitation behaviour. For an inlet concentration of 75 mg dm(-3) of NH4+-N and 25 mg dm(-3) of methanol, the organic substrate was consumed rapidly near the inlet of the reactor while most of the NH4+-N was converted in the central region. The concentration profiles within the biofilm indicated that oxygen was limiting near the inlet of the reactor while organic substrate was limiting at the middle of the reactor. Both NH4+-N and methanol became limiting near the reactor exit. Detailed analyses were carried out for a wide range of NH4+-N concentrations and for a given concentration of methanol which showed that oxygen could also become limiting even at the middle of the reactor.