Aerobic granular sludge (AGS) is an emerging technology offering an alternative wastewater treatment with a reduced footprint compared to conventional activated sludge systems. Basic understanding of AGS has mainly been obtained in lab-scale reactors fed with simple synthetic wastewaters. Yet, the properties and performances of AGS cultivated in these model systems are different from those obtained in reactors treating real wastewater. The composition of the synthetic wastewater of an AGS reactor was progressively changed from a simple wastewater containing mainly volatile fatty acids to a complex monomeric wastewater containing amino acids and glucose (1), from complex monomeric wastewater to a complex polymeric wastewater containing starch and peptone (2), and from complex monomeric back to simple wastewater (3). The microbial community of the sludge, its performances and properties were monitored. During transition (1), a clear shift from the dominant class of β-Proteobacteria to Actinobacteria was first observed, followed by a second phase where the two classes were present in high abundance. During transition (2), a slight increase of members of the Saccharibacteria phylum and the class of Sphingobacteria was noticed. After transition (3) the microbial community was similar to the initial one. The nutrient removal performances and granulation remained stable with the monomeric wastewaters. However, denitrification and granulation were impaired with the addition of polymeric compounds (2). As conclusion, changes in the influent wastewater composition led to reversible changes in the microbial communities of AGS while denitrification performances and granulation were affected by the presence of polymeric compounds in the influent wastewater