Major ion concentrations and Sr isotope ratios ( 87 Sr/ 86 Sr) were measured in rainwater samples collected at an urban site in Beijing over a period of one year. The pH value and major ion concentrations of samples varied considerably, and about 50% of the rainwater studied here were acidic rain with pH values less than 5.0. Ca 2+ and NH 4 + were the dominant cations in rainwaters and their volume weighted mean (VWM) values were 608μeql −1 (14–1781μeql −1 ) and 186μeql −1 (48–672μeql −1 ), respectively. SO 4 2− was the predominant anion with VWM value of 316μeql −1 (65–987μeql −1 ), next was NO 3 − with VWM value of 109μeql −1 (30–382μeql −1 ).Using Na as an indicator of marine origin, and Al for the terrestrial inputs, the proportions of sea salt and terrestrial elements were estimated from elemental ratios. More than 99% of Ca 2+ and 98% of SO 4 2− in rainwater samples are non-sea-salt origin. The 87 Sr/ 86 Sr ratios were used to characterize the different sources based on the data sets of this study and those from literatures. Such sources include sea salts ( 87 Sr/ 86 Sr∼0.90917), soluble soil dust minerals originating from either local or the desert and loess areas (∼0.7111), and anthropogenic sources (fertilizers, coal combustion and automobile exhausts). The high concentrations of alkaline ions (mainly Ca 2+ ) in Beijing atmosphere have played an important role to neutralize the acidity of rainwater. However, it is worth noting that there is a remarkable acidification trend of rainwater in Beijing recent years.