The simultaneous ammonium and p-hydroxybenzaldehyde (pOHBD) oxidation capacity of a nitrifying sludge was investigated in a sequencing batch reactor (SBR). At all initial pOHBD concentrations tested (25-400 mg C/L), both ammonium (100 mg NH₄⁺-N/L) and pOHBD were consumed with efficiencies of 99.2 ± 1.5% and 100 ± 1%, respectively. At pOHBD concentrations lower than 100 mg C/L, the main product of ammonium oxidation was nitrate with a yield (Y_NO3) of 0.97 ± 0.03 g NO₃^--N/g NH₄⁺-N consumed. At 200 and 400 mg pOHBD-C/L, Y_NO3 decreased to 0.78 ± 0.05 and nitrite was detected (Y_NO2 = 0.04 ± 0.01 g NO₂^--N/g NH₄⁺-N consumed). p-Hydroxybenzoate (pOHBT) was detected as product of pOHBD oxidation. pOHBT accumulation was significant in the first operation cycles at 25 mg pOHBD-C/L. Afterward, pOHBT was completely removed and no aromatic intermediates were detected. At low C/N ratio values (0.25-4), a dissimilatory nitrifying respiratory process was maintained (Y_BM = 0.03 ± 0.01 g biomass-N/g NH₄⁺-N consumed). These results show that nitrifying SBR can be successfully used for the simultaneous removal of ammonium and p-hydroxybenzaldehyde in a unique reactor. This information might be useful for treating industrial wastewaters contaminated with nitrogen and recalcitrant phenolic compounds.