Dielectrophoretic behavior of single polystyrene-carboxylate microparticles and polystyrene microparticles was studied with a quadrupole microelectrode. For positive dielectrophoresis (DEP) of polystyrene-carboxylate particle observed in the lower frequency region (less than or equal to 10 kHz), a DEP mobility coefficient (alpha) was almost proportional to the square of ac voltage (U-rms, root mean square), as was expected from standard theories of DEP. The alpha values did not depend on the particle radius (r(e)), but depended on the kind of electrolyte in the order HCl>> KCl approximate to KOH greater than or equal to tetrabutylammonium chloride, showing a specific enhancement by H+. Since this observation was against the standard theories, we introduced a DEP radius (r(DEP)) of a Debye-type function instead of the particle radius to fit the observed alpha values in the lower frequency region. The positive DEP behaviors were reproduced by a function of r(DEP) and the surface conductivity of the negatively charged particles. As for negative DEP observed in the higher frequency region (greater than or equal to 56 kHz), proportional relationships between alpha and U-rms(2) and ones between alpha and r(e)(2) were Obtained as predicted by the standard theories. The obtained r(DEP) values increased with a decrease in the ac frequency and the r(DEP) value in the HCl system was larger than those for the other systems. These suggested that the r(DEP) was controlled by the moving distance of cations swung by the applied ac electric field in the dynamic diffusion cloud around the particles.