Despite the growth in aquifer storage and recovery (ASR) as a technique for the provision of potable water supplies, quantitative data on the fate of disinfection byproducts that may be present in the injected water remain rare. This study evaluates the data from eight ASR sites in Australia and the United States that cover a wide range of source water compositions, hydrogeological environments, and operating conditions. Rates of attenuation and formation of trihalomethanes (THMs) in groundwater were determined using analytical techniques that took dilution effects into account. Half-lives varied by more than 2 orders of magnitude (e.g., <1 to >120 days for total THMs) and were both compound- and site- specific. Chloroform was most persistent, and more highly brominated compounds tended to be less persistent, as has generally been found. For any particular THM compound, much of the variability could be explained by contrasts in geochemical conditions within the aquifer since microbial degradation is the primary mechanism for THM attenuation. As such, bounds on the half-life were defined according to the redox state of the groundwater. In situ formation of some THMs in the aquifer after injection was directly observed at a number of sites, and was predicted to have taken place at all sites. The variance in formation estimates was large between the different methods used. Formation may be more common than previously thought because of the low frequency of groundwater sampling after injection and concomitant attenuation and mixing.