Chemical disinfectants employed in water and wastewater treatment can produce a variety of transformation products, including carbonyl compounds (e.g., saturated and unsaturated aldehydes and ketones). Experiments conducted under conditions relevant to chlorination at drinking water treatment plants and residual chlorine application in distribution systems indicate that alpha,beta-unsaturated carbonyl compounds readily react with free chlorine and free bromine over a wide pH range but react slowly with combined chlorine (i.e., NH2CI). For nearly all of the 11 alpha,beta-unsaturated carbonyl compounds studied, the apparent second-order rate constants for the reaction with free chlorine increased in a linear manner with hypochlorite (OCI-) concentrations, yielding species-specific second-order rate constants for the reaction with OCI- ranging from 0.21 to 12 M-1 s(-1). Predictions based on the second-order rate constants indicate that a substantial fraction (i.e., >60%) of several of the more prominent alpha,beta-unsaturated carbonyls (e.g., acrolein, crotonaldehyde) will be transformed to an appreciable extent in distribution systems by free chlorine. Products from the reaction of chlorine with acrolein, crotonaldehyde, and methyl vinyl ketone were tentatively identified using nuclear magnetic resonance (NMR) and gas chromatography coupled to high-resolution time-of-flight mass spectrometry (GC-HRT-MS). These products lacked unsaturated carbons and, in some cases, contained multiple halogens.