The objective of this study is to investigate the characteristics of δD, δ18O, and deuterium excess (d) of precipitation and water vapor in surface air in Beijing, China. The δD, δ18O, and d of atmospheric water vapor in surface air were measured continuously with an in situ technique. Much less day-to-day and diurnal variations in the vapor isotopic contents were observed in the summer monsoon season (June–August) than in the rest of the year. Outside the monsoon season, the vapor δD and δ18O showed a log linear dependence on the vapor mixing ratio, and d showed a negative correlation with the local relative humidity (RH). Both relationships were statistically significant. The vapor mixing ratio and RH were poor predictors of the vapor isotopic temporal variability during the peak summer monsoon activities. In addition, an analysis was presented of the interaction of the isotopic exchange between the vapor and the condensed phase. The δD and δ18O departure from the equilibrium state was positively correlated with RH, and the d departure from the equilibrium state was negatively correlated with RH.