Biological nitrification-denitrification is the most common process for nitrogen removal from wastewaters. During the first step, ammonia is aerobically oxidized to nitrite and then to nitrate. Subsequently, this nitrate is reduced to gaseous nitrogen by denitrifying microorganisms that use it as final electron acceptor. Shortcut biological nitrogen removal is based on the fact that nitrite is an intermediary compound in both steps: a partial nitrification up to nitrite is performed followed by nitrite denitrification. This will produce savings in aeration during the nitrification step. This research studies the effect of dissolved oxygen concentration in nitrite accumulation. An activated sludge reactor is operated under different DO levels, analyzing nitrite accumulation and ammonia removal. Results show that at 1.4 mg DO/L, 75% of nitrite accumulation takes place, with 95% of ammonia removal. Moreover, nitrite accumulation showed to be stable over more than 170 days of operation. Under these conditions, a reduction of 40% in the value of the required mass transfer coefficient K(L)a is achieved. (c) 2004 Elsevier Ltd. All rights reserved.