Abstract The strong Ames mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2 (5H)-furanone (MX) and its geometric isomer E-2-chloro-3-(dichloromethyl)-4-oxobutenoic acid (E-MX) have been shown to be present in chlorinated drinking waters. MX accounts for approximately 30% and E-MX for a few percent of the overall mutagenicity. MX and E-MX are unstable in water and undergo both pH dependent isomerization (MX⇄E-MX) and hydrolytic degradation. Alternative methods of disinfection have been found to produce mutagenicity, and MX and E-MX but to a lesser extent than disinfection with chlorine. The MX analogues 3-chloro-4-(dichlorometryl)-2 (5H)-furanone (red-MX) and 2-chloro-3-(dichlorometryl)-2-butenedioic acid (ox-MX) have also been identified in chlorinated water. However, the relatively low mutagenicity of these compounds suggests that their contribution to the overall mutagenicity of chlorinated water is of only moderate significance.