Effects of salt addition on strength and dynamics of hydrophobic interactions are investigated by molecular dynamics simulations for hydrophobic solutes in aqueous solution of various salts such as sodium chloride, ammonium chloride, and guanidinium chloride. Hydrophobic interaction is reduced by ammonium chloride, enhanced by sodium chloride, and strongly enhanced by guanidinium chloride. Addition of salts tends to delay the relaxations of hydrophobic associations by reducing water diffusivities and enhancing structuring of water. The underlying molecular mechanisms are discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.