PCM (phosphonocarboxylate mixture) has been used successfully for several years for the control of corrosion in industrial cooling systems. Along with its excellent corrosion control performance, PCM has a much better environmental profile than many competing products due to its all-organic composition. However, until recently, the use of this product has been restricted to waters of low to medium hardness when used in first generation formulations. A significant breakthrough has now extended its application to hard waters. This paper reports laboratory test results and field studies where a formulated product, based on PCM, has given outstanding performance at high calcium hardness. Keywords: Phosphonocarboxylate Mixture, PCM, Corrosion Inhibition, Mild Steel, Environment, Hard Water