Colloids play a critical role in the transport of particle-bound contaminants. Knowledge of colloids and their aggregates provides insight into contaminant transport and fate within a given aquatic environment. Here, colloids and aggregates at the sediment-water interface of Vidy Bay, Lake Geneva, Switzerland, were characterized with a combination of analytical techniques to understand their structure, size distribution, concentration, and stability (the potential for aggregation). Vidy Bay is known to be the most contaminated part of Lake Geneva, being influenced by the effluents of a municipal wastewater treatment plant. Colloids were a heterogeneous mix of inorganic constituents (diatom fragments, quartz, clay, endogenic calcite, iron oxy-hydroxides) bridged together by rigid biopolymer strands or gels. The presence of rigid biopolymers was quite significant and they were typically found to have iron oxy-hydroxides embedded within their structure. Ion chromatographic data were comparable to previous values attained for the water column of Lake Geneva; however, single particle counting results indicated the presence of a nepheloid layer in Vidy Bay. In such, the stability of colloids was likely influenced by their proximity to the sediment-water interface. Zeta potential results inferred charge neutralization and destabilization of colloids and aggregates. Self-assembly of rigid biopolymers, along with cationic bridging between rigid biopolymers and inorganic constituents readily aggregated colloids. Taken together, colloids at the sediment water interface of Vidy Bay appeared to be unstable and to play a minor role in the transport of contaminants over long distances.