Limitations in applying the concept of thermal diffusivity to coarse-weave fiber-reinforced composites are examined. Using the flash technique for measuring the apparent diffusivity, the effects of fiber fraction, fiber/matrix thermal conductivity ratio fiber orientation, and specimen length are studied. The conductivity ratio was the major factor governing the thermal behavior of the composite. The fiber fraction was the next most controlling parameter. Specimen length and fiber orientation can be used to modify the behavior of composites.