Calorimetric titrimetry reveals that cyclohexano-15-crown-5 (C15C5) forms a 1:1 complex with Mn(II) ions (2.5 mM) in propylene carbonate (PC) solution to which was added levels of water up to 134 mM. The reaction proceeds with a strong driving force (log K = 4.7-5.0) and is enthalpically driven overall, but the magnitude of the enthalpy change decreases with increasing water concentration in the case of the Mn(II) nitrate salt. A minor anion effect occurs in which the trifluoromethansulfonate ion gives slightly more negative values of Delta H and T Delta S than the nitrate ion. Values of log K, however, do not change appreciably with increasing water concentration, nor are they dependent on the anion. The crystal structure of Mn(C15C5)(CF3SO3)(2) was determined, showing that the Mn(II) ion is coordinated by a pentagonal bipyramid of oxygen atoms provided by the equatorial crown ether and apical monodentate triflate ions. Crystal data for MnC16H26O11F6S2 : monoclinic, C2/c, a = 26.925(3) Angstrom, b = 14.145(1) Angstrom, c = 13.420(1) Angstrom, beta = 100.31(2)degrees, V = 5028 Angstrom(3), Z = 8, t = 26 degrees C. For 2178 reflections with I > sigma(I), and 325 parameters, R(F) = 0.050 and R(w)(F) = 0.054.