The pressure dependence of the helix-coil transition temperature of poly[d(A-T)] has been measured in aqueous solutions of NaCl, KCl, and CsCl at concentrations between 0.02 and 1 M. In all cases the transition temperature increases with pressure. For solutions of NaCl, KCl and dilute CsCl the change in the transition temperature is linear with pressure up to 200 MPa. In more concentrated CsCl solutions the change in Tm with pressure was hyperbolic. The molar volume change of the transition (delta Vt) was calculated using the Clapeyron equation. At the lower salt concentrations, the derived values of delta Vt increase with the radius of the cation (Na+ < K+ < Cs+). At the higher salt concentrations delta Vt of poly[d(A-T)] in Na+ and K+ became equal; however, in CsCl solutions delta Vt was approximately twice as large as delta Vt in solutions containing the other two ions. In solutions of NaCl and KCl, delta Vt increased linearly with the logarithm of the salt concentration while in aqueous CsCl the concentration dependence of delta Vt was hyperbolic. The results are interpreted in terms of the role played by the radius of the cation in deciding the strength of the interactions formed with water.