A method is described for the measurement of linear thermal expansion in the plane of thin materials. The measurements are performed by a thermomechanical analysis (TMA) technique developed for polymer films, in which samples are held in a specially designed set of Invar chucks. A critical aspect of this technique is the mode in which the chucks grip the composites, as compared to the thinner, more pliable polymer films. Thermal expansions of composite materials with&agr;≥ 5 ppm/°Cwere found to be precise to within ±3% and accurate to within ±6% when dry specimens were measured at 0% relative humidity.