Loose deposits (LD) develop and accumulate in drinking water distribution systems' (DWDS) pipes and often lead to discolouration incidents, which are a major reason for customers' complaints. In addition, LD may accumulate contaminants, while discolouration may be accompanied by degradation of the water's microbiological quality. Along with iron oxides, organic compounds are well established as predominant constituents of LD. Even so, the composition and role of the latter remain unknown. Since microbial extracellular polymeric substances (EPS) are components of iron-rich flocs from many freshwater environments, the presence of EPS in LD was investigated. This was confirmed by the systematic occurrence of polysaccharides and proteins in LD samples. In addition, microscopic observations and measurements of LD water contents showed that, like their freshwater counterparts, LD have a hydrogel floc nature and fractal morphology. By providing insights on LD origin and behaviour, these findings may be important for the understanding of DWDS water discolouration phenomena.