The biogeochemical cycling of phosphorus (P) in South and Southeast Asian floodplain and delta aquifers has received insufficient attention in research studies, even though dissolved orthophosphate (PO4 3-) in this region is closely linked with the widespread contamination of groundwater with toxic arsenic (As). The overarching aim of this study was to characterize the enrichment of P in anoxic Highlights • Consistent high PO4 3conc. in groundwater of the Red River and the Bengal Delta • PO4 3represented the dominant P species in anoxic groundwater reaching 1.5 mg L-1 • PO4 3release via reductive dissolution of Fe-oxides coupled with OM mineralization • PO4 3is subject to microbial cycling, subsurface transport, and immobilization • Stable O isotope analysis in PO4 3as tool to unravel P cycling and its sources