Rain and vapor samples were collected during NOAA flights into Hurricanes Olivia (1995) and Opal (1995) and analyzed for their oxygen-18 and deuterium contents. Several marked patterns of isotope ratios are discussed and explained. Isotope ratios were much lower on average in the hurricanes than in the surroundings. They also decreased inward from the outer edge of the precipitation shields to minimum values in the regions of stratiform precipitation, and then increased to the eyewall. In Hurricane Olivia, isotope ratios decreased with time as the storm matured from 24 to 25 September, 1994. Three-dimensional trajectories constructed from airborne dual doppler radar wind analyses in Olivia showed that isotope ratios were negatively correlated with the height of origin of the precipitation. In Hurricane Opal, anomalously high isotope ratios of rain, snow, and graupel from the eyewall indicated that at least 30% of the source vapor for the precipitation derived from the sea surface in the hurricane's inner region of high winds.