The objective of this study was to assess the effects of pH, alkalinity, and hardness on the inactivation kinetics of MS2 phage with chlorine in synthetic and natural waters. Experiments characterizing the inactivation of MS2 phage with free chlorine have been performed at 20°C in carbonate buffer solution (CBS), CBS spiked with hardness, and natural surface waters containing hardness. Contrary to previous work conducted in phosphate buffer solutions, no pH effect was observed in CBS or in natural waters. This suggested that hypochlorous acid and hypochlorite ion have the same disinfection efficacy in waters buffered by the carbonate system. Additionally, the presence of calcium and magnesium at levels typical of natural surface waters of medium to high hardness was found to enhance the inactivation kinetics of MS2 phage with free chlorine by a factor of at least two. In 1 mM CBS, four logs of inactivation were achieved at a CT value of 2 mg*min/L. In CBS spiked with 1.8 mM calcium or 1.8 mM magnesium, the same inactivation was achieved at a CT value of less than 1 mg*min/L. This enhanced rate of inactivation correlated well with that found in natural surface waters with the same hardness levels. Additional experiments are being conducted to investigate similar effects with monochloramine.