To study the alkalization issue and its potential effects on high-load denitrifying system, the effects of operation mode on self-alkalization of high-load denitrifying reactor were investigated. The results showed that both the increase of substrate concentration and decrease of hydraulic retention time (HRT) can induce notable self-alkalization of high-load denitrification reactor (with the nitrogen loading rate (NLR) higher than 25 kg N m(-3) d(-1)). The effluent pH surpassed the 9.20 when the influent pH value was 7.0 +/- 0.1. The self-alkalization of denitrification process originated from the nitrate reduction, while the methanol oxidation could alleviate the self-alkalization by neutralizing OH- and setting up a buffering system of HCO3-/CO32-. At the same NLR, the self-alkalization induced by increase of substrate concentration was remarkably stronger than that induced by decrease of HRT. Keeping the nitrate concentration below inhibition concentration improved the performance of high-load reactor and alleviated the self-alkalization. (C) 2015 Elsevier Ltd. All rights reserved.