Elucidating the processes that affect particulate phosphorus (P) export from the euphotic zone and burial in sediments is important for models of global phosphorus, nitrogen and carbon cycling. We investigated dissolved inorganic Pi incorporation into particles (>0.2 µm) in the sub-euphotic zone and benthic boundary layer (BBL) of high productivity Mauritanian and Namibian shelf waters, using 33PO43- tracer experiments combined with a sequential chemical extraction analysis. Pi uptake (5.4 to19.9 nmol P L-1d-1) by particulate matter was biologically mediated (~50% into the organic fraction), and similar to estimated rates of heterotrophic growth. Thus, a substantial fraction of Pi must be recycled through a particle-associated microbial pool. Rapid adsorption of 33P in the anoxic waters of Namibia indicated the additional existence of a large pool of surface exchangeable P. Particle associated Pi recycling and adsorption may influence the export flux and ultimate fate of particle bound P in continental shelf waters.