Sevelamer hydrochloride is a cross-linked polymeric amine; it is the active ingredient of Renagel capsules. Renagel is indicated for the control of hyperphosphatemia in patients with end-stage renal disease. An in vitro phosphate-binding assay is required to measure the drug's efficacy. The assay developed for this purpose involves mixing the drug (polymer) with a solution of known phosphate concentration, filtering off the polymer-phosphate complex, and quantitating the unbound phosphate concentration by ion chromatography. The binding capacity, reported as mmol of phosphate bound g of polymer(-1), is calculated from the calculated amount of bound phosphate and the weight of polymer used. The method has been validated for accuracy, precision, linearity, range, and ruggedness.