We investigated the interactions of dissolved reactive phosphorus (DRP) and dissolved nonreactive phosphorus (DNRP) with suspended and settled aquatic particles. The sorption to minerogenic particles from an alpine catchment collected in rivers, hydropower reservoirs, and a downstream ultraoligotrophic lake was modeled using Langmuir-type isotherms. DRP and DNRP exhibited about equal affinities to particle surfaces. The sorption of dissolved species to surfaces alters the fate of P in water bodies. In spite of the small surface-binding constants, high particle concentrations enhance the sorption of P to surfaces, and, consequently, chemical analysis of DRP can substantially underestimate the potentially bioavailable P. In unpolluted rivers with high content of suspended mineral particles, e. g., triggered by heavy rain events (2 mu g DRP/L, 1.3 g/L suspended particles), P loads solely based on DRP measurements underestimate the true load of potentially bioavailable P by more than a factor of two. Modeling P sorption equilibria with a single type of surface site generates a management tool for water quality in P-limited oligotrophic systems.