Specific salt effects were studied on the quenching reaction of excited [Ru(N-N) 3 ] 2 + (N-N=2,2 ' -bipyridine (bpy), 1,10-phenanthrorine (phen)) and [Cr(bpy) 3 ] 3 + by [Cr(ox) 3 ] 3 - (ox=oxalate ion) and [Cr(mal) 3 ] 3 - (mal=malonate ion) in aqueous solutions as a function of alkali metal ions which were added for adjustment of ionic strength. The value of k q , quenching rate constants, and k 1 , energy transfer rate constant in encounter complex, is changed by the cations as the order of Li + >Na + >K + ~Rb + =<Cs + , although diffusion rate constants are not changed by the co-existing cations. Among the quenching reactions investigated in this work, a ratio of k 1 values in the aqueous solutions whose ionic strength was adjusted with LiCl and KCl, k 1 L i C l /k 1 K C l , is larger for quenching systems of closely approached donor-acceptor pair than loosely bounded pair. These results indicate that co-existing alkali cation tunes the distance between donor and acceptor in encounter complex where energy transfer occurs.