Changes in oxidization-reduction potential (ORP) and pH along with ammonia nitrogen, nitrite nitrogen and nitrate nitrogen in a twice-fed sequencing batch reactor (SBR) treating pig slurry were investigated in five experimental conditions with various aeration intensities, durations and feeding ratios. The lower the ORP value at the end of each non-aeration phase, the higher the nitrate nitrogen removal. To realize a complete denitrification, a supplement of organic matters should be conducted at a minimal loading rate of 45 mg BOD5 mg(-1) NO3–Nh(-1) for the non-aeration phase when the ORPAg/AgCl value reached -400 mV at the end of that phase. On-line monitoring results indicated that pH was more sensitive than ORP in revealing the termination of nitrification, not being interfered with variations of feeding load in the twice-fed SBR process. Additionally, both the pH variation rate and the logarithm of ORP variation rate are significantly correlated to the nitrification rate, expressed as the ammonia nitrogen decreasing rate, which could be used to estimate the nitrification rate during the aeration phase of the twice-fed SBR. In conclusion, the utilization of ORP and pH as indicators for process control of nitrogen removal had the potential to optimize pig slurry treatment by twice-fed SBR systems. (C) 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.