Of all the current potential alternatives to free residual chlorine for drinking water disinfection (ozone, chlorine dioxide, and chloramines), ozone is the most potent biocide. Chlorine dioxide is about on a par with hypochlorous acid, but in contrast to free residual chlorine, its efficiency increases substantially as pH increases in the range at which disinfection is usually applied. Chloramines are weaker biocides than hypochlorite ion, the least efficient form of free residual chlorine. Precise quantitative ranking of biocidal efficiencies is not possible because efficiencies differ with different microorganisms and experimental conditions. Laboratory studies are not always directly applicable to disinfectant use in the field.