Perturbation expansions of the effective electrical conductivity U e of any two-phase isotropic composite medium of arbitrary dimensionality d (where d = 2,3) are derived. It is shown that certain Pade approximants of a particular series representation of U e yield known rigorous bounds on the conductivity ofthe composite. The relationships between the conductivities of certain models that are exactly realized by some of these bounds and the perturbation expansions are discussed. A new expression for the conductivity of a broad class of three-dimensional dispersions of inclusions is derived. The formula for Ue , which depends upon, among other quantities, a certain three-point probability function of the composite medium, is shown to accurately predict u. of both periodic and random arrays of impenetrable spheres, for a wide range of phase conductivities and inclusion volume fractions.