PurofineÒ PFA400 is a gel-type 1 strong base anion exchange resin which because of its regeneration efficiency has particularly high operating capacity at lower regeneration levels. It is also relatively less susceptible to organic fouling than are standard gel-type strong base anion resins. Consequently higher purity treated water (or other solution) can generally be obtained. These useful advantages are obtained by way of the narrow particle size distribution. This increased capacity may be used to obtain longer runs, higher throughput and/or smaller resin beds, as required, with improved optimum rates of ion exchange fixation and regeneration. Thus economies may be made both to operating and capital costs. As is general with the PurofineÒ range, operation at higher flow rates normally detrimental to performance of conventional resins is an area which offers significant advantages.