Aqueous solubilities of ferrocene (bis(cyclopentaclienyl)iron(II), C(10)H(10)Fe) were measured at temperatures within (313 to 433) K along a 5 MPa isobar. The activity coefficients of ferrocene in aqueous solutions at saturation were obtained employing three different approximations of the solute heat capacity difference, Delta C(P2). The relative rate of increase in aqueous solubility of ferrocene with temperature, (partial derivative In x(2)/partial derivative T)(P,sigma,) is distinctly lower than the corresponding values in aromatic hydrocarbons naphthalene (C(10)H(8)) and anthracene (C(14)H(10)) possibly because of less endothermic solute water interactions because of less contact surface of the ferrocene molecule.