The response of the isotopic ratio of summer precipitation in eastern Siberia to precipitation sources is estimated by examination of stable isotope ratios in the summer precipitation and by atmospheric data analysis, based on water balance considerations. The isotopic ratio (δ18O) was analyzed in samples of precipitation that were collected daily or weekly, from May 2000 to August 2000, in the Yakutsk region of eastern Siberia. The results of the isotopic data show that during spring, the variation of isotopic ratio is controlled mainly by the Rayleigh distillation process. During summer, on the other hand, the variation of isotopic ratio bears no relation to the Rayleigh process, and isotopic variation at this time is mainly caused by changes in the sources of precipitation. This is reinforced by comparison with the analysis of atmospheric data and observed isotopic data. The comparison shows that only in the summer samples, there was a good correlation (R2 = 0.88) between heavier isotopic ratios and increases in the proportion of the moisture supply that originates from the west, which is the dominant source of precipitation (about one half of the moisture). This linear relationship indicates the summer precipitation in eastern Siberia is a composite of source water from the west with heavy isotopic ratio and other sources with light isotopic ratio. The origin of this light isotopic ratio in summer precipitation might be recycled water from the land surface. Recycled water with light isotopic ratio results from the contribution of thaw water.