Isotopic composition of atmospheric water vapour (δ18OV) was observed at a larch forest near Yakutsk in eastern Siberia during the late summer periods of 2006, 2007 and 2008. The δ18OV [and deuterium excess (d-excess)] values observed in 2006 and 2008 positively (and negatively) correlated with mixing ratio of atmospheric water vapour, whereas, in 2007 when soil was extremely wet and resulted in limitation of plant transpiration, neither correlation was found between mixing ratio and δ18OV nor d-excess. Observed results were also compared with components of atmospheric water balance calculated for a 500 × 500 km region; however, neither specific relationship between δ18OV and horizontal advection (direction) nor evapotranspiration was observed. On the other hand, obviously low δ18OV and high d-excess values were found with low mixing ratio after removal of water vapour from the atmosphere because of the process of rainout in 2006 and 2008. Assuming the δ18OV under this condition to be a background, and also assuming the δ18O of sap water in larch trees as transpired water vapour, contribution of transpiration to the atmospheric water vapour was calculated. Fraction of transpired water vapour to the atmospheric water vapour was nearly 0·8 in maximum when plant transpiration was active under warm condition. Our isotope data confirm the importance of recycling of water through transpiration of forest plants in taiga to the hydrologic cycle in eastern Siberia. Copyright © 2013 John Wiley & Sons, Ltd.