Presented at the 2014 Ocean Sciences MeetingMeasurements of d18O and dD in seawater and freshwater inform us about processes such as evaporation, precipitation, tidal mixing, submarine groundwater discharge and salt rejection during sea-ice formation. With the increasing use of laser-based systems, such as Cavity Ring-Down Spectroscopy (CRDS), field-based and shipboard isotopic measurements are now possible, but scientists remain limited by the requirement of discrete liquid water samples. Many processes, especially mixing from multiple sources, would benefit from high resolution measurements of spatial and temporal features. We demonstrate a continuous flow water sampler, coupled to a CRDS system, for real-time measurements of d18O and dD. Improving upon Munskgaard et al. (2011), the device pumps liquid water through an expanded polytetrafluoroethylene (ePTFE) membrane diffusion cell to continuously generate water vapor with active control for temperature and flow rate, to increase its stability and repeatability. This vapor is then measured by a Picarro L2130-i CRDS analyzer at a measurement interval of > 1 Hz. We will present results from lab-based testing of precision, stability and systematic errors, along with field-based trials in the Sacramento River. Ref: Munksgaard, N.C. et al. (2011), RCM, 25, 3706-3712.