Magnetic water treatment (MWT) is claimed to stabilize the solution pH, reduce corrosion of materials and inactivate micro-organisms. In the specific application swimming pool water treatment, MWT is additionally claimed to decrease chlorine demand. A laboratory-scale study has been carried out under rigorously controlled conditions based on a swimming pool water analogue. The efficacy of MWT on both chlorine consumption and on the inactivation of the key sewage indicator micro-organism Escherichia coli has been evaluated. In addition, the effect on the formation of the key disinfection by-products of chloramines and trichloromethane has been studied. On the basis of a factorial experimental plan, it was found that chlorine loss was significantly suppressed by MWT. The E. coli kill rate fur a given disinfectant dose was increased by an average of 25% as an effect of MWT as a result of the enhanced chlorine dissolution. It was also found that MWT reduced chloroform generation to some extent. On the other hand, no statistically significant direct effects on micro-organism deactivation were recorded.