Apparent molar adiabatic compressibilities and apparent molar volumes of poly[d(A-T)].poly[d(A-T)], poly(dA).poly(dT), DNA and poly(A).poly(U) in aqueous solutions were determined at 1 degree C. The change of concentration increment of the ultrasonic velocity upon replacing counter ion Cs+ by the Mg2+ ion was also determined for these polymers. The following conclusions have been made: (1) the hydration of the double helix of poly(dA).poly(dT) is remarkably larger than that of other polynucleotides; (2) the hydration of the AT pair in the B-form DNA is larger than that of the GC pair; (3) the substitution of Cs+ for Mg2+ ions as counter ions results in a decrease of hydration of the system polynucleotide plus Mg2+, and (4) the magnitude of this dehydration depends on the nucleotide sequence; the following rule is true: the lesser is a polynucleotide hydration, the larger dehydration upon changing Cs+ for Mg2+ ions in the ionic atmosphere of polynucleotide.