Stratospheric air in the vicinity of a balloon payload have been found to be contaminated with H2O mixing ratios of 10--80 ppmv during ascent, float, and slow descent at altitudes above 20 km. The contamination appeared in brief events often coincident with the occurrence of micron-sized particles and with increased temperatures. The observed H2O mixing ratios may result in ice particle production within 10--60 s in the lower stratosphere, even at temperatures around 200 K, i.e., well above the frost point of the ambient air. Thus, plumes of H2O-contaminated air must be seriously considered as an explanation for balloon-borne observations in the past, which have been interpreted as apparent thin layers of large ice particles above the frost point. These possible artifacts reduce the weight of evidence for NAT-coated ice particles in the stratosphere at temperatures above the frost point. ¿ 2001 American Geophysical Union