The thermal expansion of three isotropic metal-matrix composites, reinforced with SiC particles or microcellular foam, is measured between 25 degrees C and 325 degrees C. All three composites show initial coefficient of thermal expansion (CTE) values in agreement with the Turner model predictions, and near Schapery's lower-elastic bound for CTE. At higher temperatures, the CTE of foam-reinforced Al decreases, while that of the two particle-reinforced composites increases. These observations are interpreted las resulting from the presence of a very small fraction of microscopic voids within the infiltrated composites. This interpretation is confirmed With finite-element simulations of the influence of voids, cracks, and reinforcement convexity in two-dimensional (2-D) composites featuring: an interconnected reinforcement of SiC surrounding isolated Al phase regions, thermally cycled from an elevated processing temperature: and deforming in generalized plane strain.