Abstract. The ability to identify the origin of phosphorus is essential to effectively mitigate eutrophication of freshwater ecosystems. The oxygen isotope composition of orthophosphate (δ¹⁸O_p) has been suggested to have a significant prospective as a tracer for P entering freshwater ecosystems. The δ¹⁸O_p tracing method is, however, still in its preliminary stages and has proven challenging to implement for new practitioners. In order to achieve progress in developing the application of δ¹⁸O_p signatures as a tracing tool, there is a need to eliminate the methodological challenges involved in accurately determining δ¹⁸O_p. This technical note describes the various steps needed to concentrate and isolate orthophosphate in freshwater samples into an adequately pure analyte (Ag₃PO₄), without isotopic alteration during processing. The protocol compiles the disperse experiences from previous studies, combined with our own experience.