A sensitivity analysis was performed on a sediment-phosphate transport model by determining the phosphate sorption characteristics of separated size fractions and prepared size distributions of fine-grained (< 64 μm) river sediment typically transported in two southern Ontario streams. The Langmuir equation was used to quantify phosphate release from the sediment. Results of the study indicated that Langmuir coefficients determined separately for individual grain size fractions can be used to model sediment-phosphate interactions. Under laboratory conditions, output from the model provided conservative estimates of phosphate release/uptake from grain size distributions of sediment in fluvial systems. The study provides phosphate release data for use in models that estimate sediment-P losses from watersheds.