Bioavailable phosphorus (P) requires separate analysis because it is independent of physical and chemical P fractions present in runoff. Iron oxide-impregnated paper strips (Pi-strips) provide a simple technique for determining bioavailable P in aqueous samples. In this study, some controversial strip preparation procedures were examined and subsequently modified, and the efficiency of the technique was compared to the conventional physico-chemical fractionation approach. Pi-strips prepared using Whatman 50 filter paper rapidly drawn through iron chloride solution then immersed in ammonium hydroxide solution produced the most accurate and precise results. A strong asymptotic relationship between P uptake and the duration of shaking, coupled with rapid intra-sample transformations between P fractions, indicated that shaking for 8 h was sufficient for complete P uptake from natural water samples and standard solutions (0–250 μg P l−1). A single Pi-strip was used because multiple strips increased analytical error. The contribution to Pi-strips from adhering sediment particles was negligible for samples with low concentrations of solids but turbid samples (>0.75 mg l−1 suspended solids) were noticeably influenced by P derived from particulate material. Fractionation of agricultural runoff demonstrated a strong relationship between the dissolved inorganic (MRP and Pi-strip fraction, but only 85% of the total dissolved fraction (TDP) was retained by the Pi-strip, suggesting not all soluble P was available for biological uptake.