L.W. [2005]. Icarus 173 (2), 447-453.) and de . Icarus 173 (2), . The NASA Juno mission will include a six-channel microwave radiometer system (MWR) operating in the 1.3-50 cm wavelength range in order to retrieve water vapor abundances from the microwave signature of Jupiter (see, e.g., . The Juno new frontiers mission. Tech. Rep. IAC-05-A3.2.A.04, California Institute of Technology). In order to accurately interpret data from such observations, nearly 2000 laboratory measurements of the microwave opacity of H 2 O vapor in a H 2 /He atmosphere have been conducted in the 5-21 cm wavelength range (1.4-6 GHz) at pressures from 30 mbars to 101 bars and at temperatures from 330 to 525 K. The mole fraction of H 2 O (at maximum pressure) ranged from 0.19% to 3.6% with some additional measurements of pure H 2 O. These results have enabled development of the first model for the opacity of gaseous H 2 O in a H 2 /He atmosphere under jovian conditions developed from actual laboratory data. The new model is based on a terrestrial model of . Radio Science 33, 919-928), with substantial modifications to reflect the effects of jovian conditions. The new model for water vapor opacity dramatically outperforms previous models and will provide reliable results for temperatures from 300 to 525 K, at pressures up to 100 bars and at frequencies up to 6 GHz. These results will significantly reduce the uncertainties in the retrieval of jovian atmospheric water vapor abundances from the microwave radiometric measurements from the upcoming NASA Juno mission, as well as provide a clearer understanding of the role deep atmospheric water vapor may play in the decimeter-wavelength spectrum of Saturn.