Interparticle interactions affecting the stability of sludge floes taken from laboratory-scale sequencing batch reactors at different solids retention times (SRTs) were investigated in batch experiments by varying the pH, ionic strength, cation valence, and urea and ethylenediaminetetraacetate concentrations of suspending solutions. The ultrastructure of sludge floe surfaces was observed by transmission electron microscopy. Changes in dissociation constants of sludge flocs under different conditions indicated that ionic interactions and hydrogen bonds held floes together and compensated for the negative influence of electrostatic interactions on the stability of sludge floes. Ionic interactions and hydrogen bonds were two dominant forces that maintained the stability of sludge floes at lower SRTs; other mechanisms, such as physical enmeshment and van der Waals and/or hydrophobic interactions, were more important in controling the stability of sludge floes at higher SRTs. Sludge floes at higher SRTs (16 and 20 days) were physically more stable than those at lower SRTs (4 and 9 days). A conceptual model of floe structure, based on interparticle interactions, for describing the stability of sludge flocs is proposed. The floe matrix is proposed to consist of two physically distinct regions that are defined by the arrangement of extracellular polymeric substances (EPS). These are likely to be differentially affected by the agents applied to manipulate interparticle forces. Thus, the heterogeneity in the packing of and the type of EPS reflects the stability of the floe.