The kinetics of the reaction of chlorine nitrate (ClONO2) with water vapor have been investigated in large-volume (2600-L and 5800-L) Teflon or Teflon-coated environmental chambers to assess the importance of this reaction as a loss process for odd nitrogen in the stratosphere. These data show that the upper limit rate constant for the homogeneous gas phase reaction between ClONO2 and H2O is <2¿10-21 cm3 molecule-1 s-1 at 298¿1 K. With this rate constant, the homogeneous reaction of ClONO2 with water vapor will be of negligible importance under stratospheric conditions.