The effect of porosity on the residual thermal stresses generated on cooling from the fabrication temperature in ceramic composites and laminates is calculated. It is suggested that the stresses can be tailored via the porosity effect on Young's modulus only, since porosity does not affect the thermal expansion behaviour of the composite constituents. Using a previously derived Young's modulus-porosity correlation, which takes into account the effects of pore shape and orientation, the thermal stresses for a number of composite systems were calculated. For matrix-type composite materials, it is shown that by controlled variation of the amount and shape of porosity in the matrix, it should be possible to obtain the desired level of internal thermal stresses, while keeping the highest possible value of the composite Young's modulus and strength. For composites with laminate structure, both porosity incorporation in one of the constituents and control of the layer thickness should lead to the optimization of laminate design.