Many factors, including pH, temperature, make-up water quality, and heat exchanger metallurgy, influence the performance of polymeric inhibitors in treating recirculating cooling water systems. The availability of good quality make-up water and the imposition of stringent wastewater discharge regulations have forced many cooling systems to operate at increasingly higher cycles of concentration. The severity of these operating conditions often results in waters that have higher scale formation potential. Presented here is information regarding the influence of impurities, such as suspended matter and metal ions, on the performance of polymeric inhibitors used in phosphate-based treatment programs. Keywords: calcium phosphate, cooling water, polymers, scale, metal ions