The waste stabilization pond (WSP) systems are useful for the treatment of wastewater. Despite a good knowledge of these systems, the dynamics of phosphorus elimination in these systems are not well known, especially concerning the forms of phosphorus in the sediment. Phosphorus dynamics were studied with data collected from the three stabilization ponds of the Mèze (France) system. A conceptual model based on theoretical considerations was developed, and applied to each pond. The percentage of observed variation described by these models ranged between 46.2% and 74.1%. The results showed that the principal variables affecting orthophosphate in WSP systems were phosphorus input and particulate phosphorus. When the model was applied to high rate algal pond (HRAP) data, the role of photosynthesis on pH increase and P precipitation was clear. Sediments sampled in the three facultative ponds were characterised with a fractionation scheme, currently used for sediments of natural aquatic ecosystems. Inorganic phosphorus accounted for 92-94% of total sediment phosphorus, with 57–59% bound to iron hydroxides and 33–37% bound to calcium. Surprisingly, organic phosphorus represented only between 6 and 8%. Polyphosphate was detected. Phosphorus concentration was greatest in the sediment from the third pond, where an aerobic layer at the sediment surface prevented phosphorus release.