The transport properties of cement-based composites, including solute diffusivity, electrical conductivity and water permeability, are regarded as durability indicators of cement-based composites. These transport properties are closely related to the microstructure, or rather to the pore structure of materials. Among all the microstructural aspects, the interfacial transition zone (ITZ) between the cement paste matrix and aggregates, and the air voids are believed to play an essential role in the transport properties. However, their impacts on the transport properties are difficult to be quantified. This paper develops a closed-form four-phase micromechanical model accounting for the local properties of ITZ and the saturation states of air voids. The effects of ITZ and air voids on the transport properties of cement-based composites are addressed quantitatively in the model. The Katz-Thompson equation is reinterpreted by the model in particular. It is shown that the local properties of ITZ and volume fraction of aggregates act mutually on the overall transport properties, the influence of air voids depends significantly on the water saturation, and a critical saturation degree is found to be 1/3.