The Smoluchowski theory has been widely accepted as the basic theory to estimate the rapid coagulation rate of colloidal particles in electrolyte solutions. However, because the size and specificity of molecules and ions are not taken into account, the theory is applicable only if the particle size is large enough to neglect the effects caused by the structured layers composed of water molecules, ions, and hydrated ions adsorbed on the colloidal surface. In the present study, the rapid coagulation rates of silica nanoparticles in concentrated chloride and potassium solutions were measured by using a low-angle light-scattering apparatus, and the dependence of the experimental value of rapid coagulation rate, K-E(R), on the particle diameter, D-P, and also on the Gibbs free energy of hydration of ions, Delta G(hyd), was investigated extensively. The following were found. (1) When the particle size was small enough, the value of K-E(R) reduced exponentially not only with the decreasing particle size but also with the increasing value of (-Delta G(hyd)) of cations (counterions) in the case of chloride solutions and with that of anions (coions) in the case of potassium solutions. (2) Silica nanoparticles of D-P less than or similar to 70 rim in 1 M KNO3 and KSCN solutions did not coagulate at all, although they coagulated at D-p greater than or similar to 100 nm as in the other potassium solutions. These unexpected phenomena were explained by the proposed mechanisms.