Abstract: Water at a very high pressure (e.g. fifty thousand psi) is directed through a nozzle orifice to produce a coherent high velocity water jet particularly adapted for cutting. Prior to pressurizing the water and directing it through the nozzle, the water is treated to remove dissolved solids, desirably to an extremely low level to provide very pure water (e.g. where the total dissolved solids are as little as ten parts per million or lower). This is done by directing the water through several ion exchange reactions to precipitate the solids which are then removed by filtering. The operating life of the nozzle is greatly extended by this process.