Ultrasonic velocities in aqueous solutions of some metal acetates, monochloroacetates and trichloroacetates, and the respective acids have been measured at 1 MHz frequency using the pulse technique. The ultrasonic velocity, adiabatic compressibility and apparent molal compressibility were measured as a function of concentration. The apparent molal compressibility values at infinite dilution were calculated and used to determine the hydration numbers. The hydration numbers obtained are, CH3COOH(HP)=3·1, NiP2=15·0, CuP2=8·6, LaP3=18·3, PrP3=20·3, NdP3=16·5, ClCH2COOH(HQ)=3·5, NiQ2=14·0 CuQ2=7·6, MgQ2=9·3, CaQ2=9·4, BaQ2=17·5, LaQ3=19·5, PrQ3=18·5, NdQ3=18·0, YQ3=20·5, Cl3CCOOH(HR)=6·3, NiR2=11·5, CuR2=6·3, MgR2=8·8, CaR2=9·8, BaR2=13·0, LaR3=28·8, PrR3=30·0, NdR3=27·8 and YR3=31·3. The low hydration numbers obtained for most of the salts of divalent metal ions are probably due to incomplete dissociation in solution.