Intermediate layer water masses are defined according to temperature-salinity relationships derived from conductivity-temperature-depth (CTD) observations measured during four 1990--1991 Western Tropical Atlantic Experiment (WESTRAX) hydrographic surveys. The intermediate layer, bounded by density surfaces of sigma theta 26.00 and 27.65 (approximately 150 and 1300 m deep, respectively), is conveniently divided into upper and lower layers by the relatively low salinity Antarctic Intermediate Water (AAIW) which is centered at sigma theta 27.25 (approximately 700 m deep). Approximately 60¿5% of the region's waters are traced to a southern hemisphere origin, indicating the importance of AAIW in the western tropical Atlantic's water mass structure. The southern source water masses, South Atlantic Central Water and AAIW, enter the WESTRAX region (west of 44¿W and between the equator and 9¿N) as part of the subthermocline North Brazil Current. Depending on the season, all or part of these southern waters retroflect anticyclonically through the region and flow eastward into the North Equatorial Undercurrent. The primary northern source water mass, North Atlantic Central Water, enters the northeastern corner of the WESTRAX region as part of a cyclonic branch of the North Equatorial Current (NEC) and converges with the southern water. This meeting produces mixture water masses which make up 45¿4% of the region in volume and are predominantly of a southern nature. The patterns of the mixture water masses which fill the areas between the source water masses suggest the importance of lateral mixing in this ocean region. Further, some of the mixture water in the upper layer appears to be part of the NEC, suggesting southern water recirculation in the tropical Atlantic gyre. A time dependent water mass box model of advective and mixing transports is used to suggest that lateral mixing dominates vertical mixing by a ratio of approximately 10 to 1. Typical box model results for the fall-winter 1990--1991 period indicate that 13 Sv of mixture water masses are produced through mixing (a sum of 9 Sv and 4 Sv from southern and northern source water masses, respectively), while a net 17 Sv of mixture water masses are exported from the region. ¿ American Geophysical Union 1996