Spectra of the binary complexes of water and ammonia molecules in the OH stretching region have been studied in helium droplets. The infrared intensity in the hydrogen bond bridge stretching mode was found to be enhanced by a factor of 2.5 with respect to the nu(3) band of single water molecules. This enhancement factor is significantly smaller than that obtained in recent theoretical calculations. The observed large band frequency shifts are similar to those measured in other rare gas matrices indicating the strong hydrogen bonding in the complex. (C) 2007 Elsevier B.V. All rights reserved.