The present work, which deals with acoustic velocity at 500 kc in aqueous solutions of LiOH, NaOH, KOH, andNH4OHand results derived therefrom, follows that which was recently reported concerning solutions of certain sulfates. It was found that the velocityvstemperature curves rise and are displaced toward lower temperatures with increase in concentration. The specific acoustic impedance varies linearly with concentration at 20°C. Adiabatic compressibilities are represented by the empirical 3/2‐power law. Results forNH4OHare at variance with those for the other hydroxides. The relative association declines with increase in concentration in solutions ofNH4OHwhereas it increases for the other hydroxides. From the data the apparent molal compressibilities were computed for the temperature range 15° to 35°C. Based on values of the apparent molal compressibilities at infinite dilution the solvation appears to be about six water molecules per ion.