Chlorofluorocarbon (CFC) measurements were made in the northwestern Indian Ocean in August--September 1995 as part of the World Ocean Circulation Experiment Hydrographic Program Section I1. The data in the Gulf of Aden section from this expedition provide information on the mixing processes that affect Red Sea Water (RSW) as it flows from its source into the open northwestern Indian Ocean. Combining measurements by Rhein et al. <1997> with the outcome from a two-end-member mixing model results in an estimate of 65--70% saturation for the CFCs in the RSW end-member at Bab el Mandeb. A boundary condition that may be more applicable to ocean models is given by the CFC content of RSW after its initial diapycnal descent along the continental slope in the western Gulf of Aden. Entrainment of water containing lower amounts of CFCs produces RSW that has CFC saturations of 20--50% once RSW has settled at its depth of neutral buoyancy. Apparent pCFC ages at Bab el Mandeb and after the separation of RSW from the bottom are 12--13 and 18--27 years, respectively. As RSW continues eastward through the Gulf of Aden as an isopycnal flow, horizontal eddy diffusion plays an important role in further diluting RSW, which is characterized by a maximum in salinity and a positive anomaly in CFC concentrations. Horizontal eddy diffusion parameters estimated from tracer conservation in a box model of the central and eastern Gulf of Aden range from 0.2¿104 to 2.0¿104 m2 s-1. As other observations show, eddy diffusion is presumably enhanced in this part of the ocean. ¿ 1999 American Geophysical Union