The upper basin of Effingham Inlet possesses permanently anoxic bottom waters, with a water column redox transition zone typically occurring at least 40 m above the sediment-water interface. During our sampling campaign in April and July 2007, this redox transition zone was associated with sharp peaks in a variety of parameters, including soluble reactive phosphorus (SRP) and total particulate phosphorus (TPP). Based on sequential extraction results, TPP maxima exhibited preferential accumulation of an operationally defined class of loosely adsorbed organic phosphorus (P), which may contain a substantial fraction of polyphosphate (poly-P). This poly-P may furthermore be involved in the redox-dependent remobilization of SRP. For example, direct fluorometric analysis of poly-P content revealed that particulate inorganic poly-P was present at concentrations ranging from 1 to 9 nM P within and several meters above the TPP maximum. Below the depth of 1% oxygen saturation, however, particulate inorganic poly-P was undetectable (<0.8 nM in situ). Assuming this concentration profile reflects the remineralization of inorganic poly-P to SRP across the redox transition, inorganic poly-P degradation accounted for as much as 4 ± 3% (average ± standard deviation) to 9 ± 8% of the vertical turbulent diffusive SRP flux. This finding is a conservative estimate due in part to sample storage effects associated with our analysis of poly-P content. By comparison, iron-linked P cycling accounted for at most 65 ± 33% of the diffusive SRP flux, leaving ∼25% unaccounted for. Thus, while redox-sensitive poly-P remineralization in Effingham Inlet appears modest based on our direct conservative estimate, it may be higher from a mass balance viewpoint. Poly-P cycling may therefore be an overlooked mechanism for the redox-sensitive cycling of P in some hypoxic/anoxic boundaries, especially iron-poor marine oxygen minimum zones.