The goal of this study was to investigate the effect of transmembrane gas pressure (P (g)) on the specific ammonium removal rate in a membrane-aerated biofilm reactor (MABR). Our experimental results show that the specific ammonium removal rate increased from 4.98 to 9.26 gN m(-2) day(-1) when P (g) increased from 2 to 20 kPa in an MABR with a biofilm thickness of approximately 600 mu m. However, this improvement was not linear; there was a threshold of P (g) separating the stronger and weaker effects of P (g). The ammonium removal rate was improved less significantly when P (g) was over the threshold, indicating that there is an optimal threshold of P (g) for maximizing ammonium removal in an MABR. The change in oxygen penetration depth (d (p)) is less sensitive to P (g) in the ammonia-oxidizing active layer than in the inactive layer in membrane-aerated biofilm. The location of the P (g) threshold is at the same point as the thickness of the active layer on the curve of d (p) versus P (g); thus, the active layer thickness and the optimal P (g) can be determined on the basis of the changes in the slope of d (p) to P (g).