Enthalpies of association of nitrate and chlorate with manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) have been determined by direct calorimetry in an aqueous medium at 25 °C and ionic strength I= 1 mol dm–3. The corresponding entropy values have been obtained by using known values of the association constants. The thermodynamic parameters suggest formation of ion pairs of the outer-sphere type, and also indicate the importance of electrostatics in determining the association processes.