In this study, six N-acyl-homoserine lactone (AHL) molecules (C-6-HSL, C-8-HSL, C-10-HSL, 3-oxo-C-6-HSL, 3-oxo- C-8-HSL and 3-oxo-C-10-HSL) were each dosed into a bioreactor and seeded using autotrophic nitrifying sludge (ANS). The effects of the AHLs on cell adhesion, nitrification and sludge granulation were investigated. The results indicated that the efficiencies of cell adhesion and ammonia removal both had a close correlation with the side chain length and beta position substituent group of the AHLs. The best-performing AHL in terms of accelerating bacterial attached-growth was 3-oxo-C-6-HSL, whereas C-6-HSL outperformed the others in terms of the ammonia degradation rate. The addition of 3-oxo-C6HSL or C-6-HSL increased the biomass growth rate, microbial activity, extracellular proteins and nitrifying bacteria, which can accelerate the formation of nitrifying granules. Consequently, selecting AHL molecules that could improve bacteria in attached-growth mode and nitrification efficiency simultaneously will most likely facilitate the rapid granulation of nitrifying sludge. (C) 2015 Elsevier Ltd. All rights reserved.