The ultrasonic absorption coefficients in water solutions of magnesium sulfate were measured at 30 Mc as a function of temperature and concentration. Temperatures from 0 to 30°C and concentrations from 0.05 to 2.00 molarity were used. Measured data were used to calculateQ, the absorption cross section per molecule of magnesium sulfate at several temperatures.Qis defined by (2αM/ν2)/NwhereNis the number of molecules of magnesium sulfate per cc, ν is the frequency, and αMis the amplitude absorption coefficient caused by the magnesium sulfate. These are constant for concentrations up to 1‐molarity but decrease by 50 percent for 2‐molarity concentration. The constant values ofQ×1037, 25 at 25°C, 30 at 20°C and 37 at 15°C indicate an activation energy for the magnesium sulfate absorption process at 30 Mc of 6.8 kcal/mole°K.