We present experimental results on electrorheological behavior under an ac electric field, and dielectric properties of colloidal suspensions composed of aggregated titanium dioxide nanoparticles dispersed in insulating PDMS silicon oil, as a function of adsorbed acetylacetone (Acac) dipolar molecules on TiO2 surface. The results show that the elastic modulus G' of TiO2-Acac/PDMS ER fluid decreases when the electric field frequency is increased gradually from 10 to 10(4) Hz. In the range of tested electric field frequencies, we observe that the dielectric properties are given essentially by an interfacial polarization which is maximal at low frequency. At low frequency (v = 10 Hz), G' increases with the increasing Acac/TiO2 surface coverage theta from 0 to 0.45, due to the high polarization of the particles caused by the increase of the charge carriers that move to the interface between TiO2 particles and the insulating medium. For theta = 0.35, G' reaches 1.80 MPa which is 3.6 times greater than that of pure TiO2/PDMS ER fluid. However, G' decreases by increasing theta beyond 0.45; in such cases, the mismatch on dielectric constant and conductivity between dispersed phase (TiO2) and insulating medium (PDMS) begins to fall as theta increases because of the increase of Acac dipoles remaining free in PDMS. The performance of TiO2-Acac/PDMS ER fluid at low frequency was validated by a reversibility test where elastic modulus G' increases dramatically and returns to its initial state as soon as several cycles of a switching electric field were applied between 0 and 1 kV/mm successively.