We report experimental and theoretical results on the steady and oscillatory shear viscoelasticity of a model electrorheological fluid. Details of the fluid synthesis, via the nucleation and growth of monodisperse silica spheres, and light scattering studies of the spheres are also presented. The shear-thinning viscosity mu shows a power-law dependence mu similar to gamma(-Delta) on the inverse strain rate gamma, with Delta in the range 0.68-0.93. Likewise, the frequency-dependent viscosity mu(omega) shows a power-law dependence mu(omega) similar to omega(-($) over bar Delta) with ($) over bar Delta in the range 0.73-1.0. A theoretical treatment of steady-state cluster formation in applied electric and shear fields predicts the observed power-law dependences of the steady and oscillatory shear viscosities, albeit with exponents Delta = ($) over bar Delta = 2/3.