The nonlinearity parameterB/Aof seawater from 25%oto 40%ohas been calculated in the temperature range from 0° to 35 °C and at pressure of up to 1000 bars by use of the acoustic and thermodynamic approaches. Their agreement with an accuracy of 2% is within ±0.1 over most of the range from 0° to 35 °C up to 800 bars. The ratioB/Aat each concentration increases monotonically with pressure in the range covered. The increase rate at higher temperatures becomes slower than that at low temperatures. It is found that theB/Aat low pressure increases with increasing temperature, and at higher pressures decreases. However, the quantityB/Aat moderate pressure near 500 bars is relatively insensitive to temperature changes and then reveals a small temperature minimum. Although theB/Aat a fixedPandTincreases with increasing salinity, the increment ofB/Adecreases as pressure increases. To find a relationship betweenB/AandTorP, we obtained the thermodynamic expressions for [∂(B/A)/∂T]Pand [∂(B/A)/∂P]T, respectively.