Ultrasonic velocities and their variation with concentration have been measured for aqueous electrolytes of various valence types at 25°. The experimental adiabatic compressibilities of solutions obtained from velocity measurements are compared with those calculated on the basis of simple additivity law making appropriate corrections for hydrated water. The agreement between the experimental and calculated compressibilities is satisfactory. The changes m hydration number with concentration as calculated using Barnartt's formula have been attributed to water layers intermediate between The incompressible first shell and the bulk water.