Three transatlantic hydrographic sections across 24°N through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics. In the main thermocline, there has been a steady increase in salinity from 1957 to 1981 to 1992. The salinity at a given potential temperature between 12°C and 17°C has increased at a nearly constant rate of 0.010 per decade; at constant potential density, salinity and temperature both increased at rates of 0.018 per decade and 0.065°C per decade. In the intermediate waters between 800 and 1500 dbar, salinity has increased as well, on average by as much as 0.025 from 1981 to 1992, either on constant potential temperature or constant density surfaces. These changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure. Throughout the water column down to 2000 dbar and across the zonal extent of the 24°N ... Abstract Three transatlantic hydrographic sections across 24°N through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics. In the main thermocline, there has been a steady increase in salinity from 1957 to 1981 to 1992. The salinity at a given potential temperature between 12°C and 17°C has increased at a nearly constant rate of 0.010 per decade; at constant potential density, salinity and temperature both increased at rates of 0.018 per decade and 0.065°C per decade. In the intermediate waters between 800 and 1500 dbar, salinity has increased as well, on average by as much as 0.025 from 1981 to 1992, either on constant potential temperature or constant density surfaces. These changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure. Throughout the water column down to 2000 dbar and across the zonal extent of the 24°N ...