A new, two-step method for the synthesis of porous mullite-cordierite composites with controllable coefficients of thermal expansion (CTE) values is reported. In the first step, a porous structure of interconnected acicular mullite grains with high aspect ratios is formed by using SiF4 gas at elevated temperatures. This intermediate contains particles of MgF2 and cristobalite that are attached to the mullite grains. In the second step, MgF2 and cristobalite react with parts of the acicular mullite structure, and they form cordierite, while maintaining the acicular nature of the resulting mullite-cordierite composite. CTE of mullite-cordierite composites synthesized using this method decrease about linearly with increasing cordierite content, and porosities range from 49% in cordierite-rich composite to 54% in a mullite-rich composite. The CTE value of a desired porous composite can be easily controlled by choosing the appropriate ratios of reactants Al2O3, SiO2, and MgO, which result in the desired mullite-to-cordierite ratio with its characteristic CTE value.