The ability to identify the origin of phosphorus and understand processes controlling P cycling is essential for designing effective mitigation and restoration of eutrophic freshwater ecosystems. The oxygen isotope composition of orthophosphate (δ¹⁸O_p) has significant potential as a tracer for P entering freshwater ecosystems. However, methods of analysis of δ¹⁸O_p are still in their preliminary stages and have 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 protocol article describes the various steps needed to concentrate and isolate orthophosphate in freshwater samples into an adequately pure Ag₃PO₄ analyte, without isotopic alteration during processing. The protocol compiles the disperse experiences from previous studies, combined with our own experience. The twofold aim of the paper is toprovide a baseline for an increasing standardisation of the silver phosphate purification method associated with analysis of the oxygen isotope composition of orthophosphate (δ¹⁸O_p), and to foster new research in the applicability of δ¹⁸O_p signatures for P source tracing in catchment science.