Biofilm-internal and external mass transfer resistance was investigated in laboratory-scale nitrifying suspended carrier reactors (SCR), demonstrating the importance of these factors for these increasingly popular reactor systems. Controlled respirometric experiments revealed that oxygen mass transfer resistance regulated the process performance up to a DO concentration of 20 mg L(-1). External mass transfer exerts significant control over the overall reaction rate, thus biofilm models must adequately account for this resistance. Whilst carrier type and characteristics have some influence, biofilm structure seems primarily responsible for differences in mass transfer and nitrification performance. Heterogeneous biofilms grown under high ammonium loadings had much greater area-specific rates than the gel-like biofilms sourced from low loaded systems. Being a mass-transfer controlled process, the overall reaction rate of these SCR systems could be immediately increased by elevating the DO above normal operating levels (up to 20 mg L(-1)). Long-term oxygen deficiency in the lower biofilm sections does not negatively affect the biomass activity. (C) 2008 Elsevier Ltd. All rights reserved.