An experimental and theoretical study of the effect of 90° ply cracking on the thermal expansion coefficients of crossply laminates has been carried out. It has been found experimentally that reductions in the coefficient of thermal expansion of up to 50% are caused by 90° ply cracks induced mechanically, although considerable care is needed in the experimentation. This behavior was modeled using a simple shear-lag analysis, and the resulting analytical expressions are compared with other approaches available in the literature. The growth of matrix cracks in a model GFRP system under severe thermal cycling (77 to 373 K) is investigated. The changes in expansion coefficient are affected by the growth of 0° ply cracks in addition to the 90° ply cracks. The crack growth rate/cyclic strain energy release rate range data are compared with those reported previously for mechanical fatigue cycling of similar material. The two data sets are consistent if plotted in terms of a fracture mechanics parameter which aims to account for the temperature dependence of material properties.