The Romanche and Chain Fracture Zones (RFZ and CFZ) are known as pathways for the Lower North Atlantic Deep Water (LNADW) and the Antarctic Bottom Water (AABW) from the western trough to the eastern trough of the equatorial Atlantic Ocean across the Mid-Atlantic Ridge. We report hydrographic observations describing the evolution of the LNADW and AABW properties during their transit through the equatorial fracture zones. To the west of the RFZ and CFZ sills, the LNADW and AABW cores are separated by a deep thermocline (pycnocline) which marks the transition between the two water masses. The deep thermocline is eroded eastward and vanishes in the eastern basin. This zonal asymmetry is the signature of a zonal baroclinic pressure gradient associated in each fracture zone with eastward flows of LNADW and AABW below 3800 m depth. During the eastward transit through the RFZ, the near-bottom AABW becomes less dense (saltier ~0.07¿ and warmer ~0.7 ¿C), richer in oxygen ~0.3 mL L-1, but less rich in silicate ~30 μmol L-1. Simultaneously, the LNADW core becomes fresher and colder at a given density, and the oxygen maximum characterizing the LNADW core weakens by ~0.2 mL L-1. We argue that the property changes within the AABW and LNADW are due to intense vertical mixing. In addition to vertical mixing with the AABW, it seems likely that isopycnal mixing of the LNADW with the Eastern Basin Deep Water contributes to the property evolution of the LNADW exiting the RFZ. In the CFZ, evidence is also given of mixing occuring within the AABW and the LNADW. A sudden change in the AABW properties observed across the CFZ main sill suggests the blocking of the eastward progression of the densest AABW by the CFZ main sill. However, this dense water can reach the eastern basin by flowing northeastward through valleys, the sills of which are deeper than thoses of the CFZ. Finally, a simple hydraulic control model applied to the RFZ and CFZ flows predicts a transport of 1.4 (0.1) 106 m3 s-1 of water colder than 1.9 ¿C across the RFZ (CFZ) main sill.¿ 1997 American Geophysical Union