Retention ponds are designed to settle particulate phosphorus in the sediments. However, internal loading mechanisms can release bound phosphorus from sediments into the water column, leading to eutrophication conditions in the pond and downstream waterbodies. To understand these mechanisms, twentyone intact sediment and water cores were extracted from four stormwater ponds. Each mesocosm was subjected to different redox conditions and monitored over 30 days. The water column orthophosphate concentrations are used to obtain an overall sediment phosphorus release rate. Phosphorus fractions in the sediment are analyzed to better understand the role of pond sediment characteristics in phosphorus release. Redox sensitive and organic phosphorus concentrations correlated with phosphorus release from the sediments, highlighting the important influence of sediment composition in ponds. These relationships can be used to quantify phosphorus release rates in ponds by fitting regressions to oxygen consumption and sediment P fractions versus the measured phosphorus flux in the mesocosms.