Four kinds of neutral salts with different cations, LiCl, NaCl, KCl, and CsCl, were added to the dye bath to accurately understand the effect of cations on the reactive dyeing of silk with C. I. Reactive Black 5. The cations of salts added lowered the negative surface potential of the silk, improving equilibrium adsorption and the accessibility of the dyestuff to the fiber greatly and speeding up the dyeing rate in the order of $Li^+>Na^+>K^+>Cs^+$. The activation energy$(E_a)$ for the dyeing was in the order of$Li^+>Na^+>K^+>Cs^+$ but the activation free energy$(Delta{G}^*)$, or the real energy barrier for the reaction, was in the order of $Li^+>Na^+>K^+>Cs^+$ because the degree of the contribution of E$^{a}$ to the activation entropy$(Delta{S}^*)$ was $Li^+>Na^+>K^+>Cs^+$. It was found from this result that LiCl had the strongest lowering effect on the negative surface potential of silk. The decrease in $Delta{S}^*$ should be attributed to the loosely bonded activated complex of dyestufffs, cations and fiber molecules at transition state. It was clarified from the Bronsted equation that salts had the ionic strength effect and the specific salt effect on the reactive dyeing.