The effects of electrolytes (symmetric and antisymmetric) on the phase behavior, the apparent cloud point, the zeta-potential, the particle size, and the surface charge density of an oil-in-water type (O/W type) microemulsion forming in solutions of a nonionic surfactant (tetraethylene glycol monodecyl ether, C-10-POE(4))/n-decane/brine have been examined. Additions of NaSCN and/or Ca(SCN)(2) result in increasing the temperature where the O/W-type microemulsions formed, while additions of NaCl, NaNO3, CaCl2, and/or NH2CONH2 decrease the temperature. When the concentrations of NaSCN and/or Ca(SCN)(2) increase, the apparent cloud points of the O/W-type microemulsion increase, while there is a decrease in the particle sizes of the microemulsion droplets. On the other hand, as the concentrations of NaCl, NaNO3, CaCl2, or NH2CONH2 increase, the apparent cloud point of the microemulsion decreases slightly, but their particle sizes increase. The electrophoretic mobilities of the microemulsion droplets can be measured only in the systems containing NaSCN or Ca(SCN)(2). Moreover, the zeta-potential and the surface charge density show negative values at a given salinity, though C(10)POE(4) does not possess a charged component in a molecule. Applying formulas described in this article, the adsorption of anion (SCN-) to the hydrophilic moieties of C(10)POE(4) microemulsions is shown to be greater than those of cations (Na+ or Ca2+). The binding constants (K) of anion and cation to the hydrophilic groups of C(10)POE(4) microemulsions are found to be K-Na = 0 M(-1), K-Ca = 0.2 M(-1), 10.0 M(-1) < K-SCN < 12.0 M(-1) and the adsorption density of C(10)POE(4) molecules on the oil/water interface (N) is found to be N = 0.3 nm(-2); the surfactant (C(10)POE(4)) numbers per O/W-type microemulsion droplet decrease (a few thousands to a few hundreds) with the increasing concentration of NaSCN and/or Ca(SCN)(2).