A relation for an adequate pore fraction needed to obtain residual Si and C free composites via reactive Si-X alloy infiltration is presented. The volume ratio of SiC and carbonaceous phase, the composition of the infiltrating liquid and the apparent density of the preform are used as design entities. The approach allows identifying combinations of these design entities leading to desirable microstructures, e.g. those free of residual silicon or free of excess graphite. The approach gives further access to important post infiltration characteristics like propensity of the various phases. An idealising mathematical model describing the reactive flow of Si-X alloy in a single micron sized capillary channel of carbon as well as in carbonaceous preforms is presented. The model is further expanded to evaluate the infiltration depth in porous carbonaceous preform for a given composition of Si-X alloy and infiltration temperature. The model is presented for both isothermal and non-isothermal cases. The analysis is formulated in general terms and is hence applicable to a large variety of Si-C-refractory metal systems of potential interest.