On the basis of achieving shortcut nitrification in a lab-scale SBR, the effects of constant pH and unsteady pH at different free ammonia concentrations on shortcut nitrification for landfill leachate treatment was investigated. The results indicate that under the condition of DO of 0.5 +/- 0.2 mg/L and temperature of 30 +/- 2 A degrees C, the absolute value of nitrite accumulation increased significantly with the increase in free ammonia (FA) concentration from 5.30 to 48.67 mg/L; however, the nitrite accumulation rate remained almost constant at a constant pH of 8.0 A +/- 0.1. Ammonia oxidation and the nitrite accumulation become slow with the pH decreased from 8.0 A +/- 0.1 to 7.5 A +/- 0.2, and the activities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were severely inhibited when the pH further decreased to 6.5. More importantly, this study confirmed that the pH decrease from 8.0 to 6.5 within a short time exhibited significant negative effect on the ammonia oxidation rather than the FA concentration.