Neutron reflectivity measurements on deuterated polystyrene (dPS) thin films reveal a strong dependence of the coefficient of thermal expansion (CTE) on both the thickness of the film and the specific confinement boundaries. A direct comparison is made between films on low energy, fluorinated polyimide (fPI) substrates with a free surface (bilayer) and the same films after capping the bilayer with another fPI layer (trilayer). For bilayers, a significant decrease in alpha (r), the CTE above the bulk glass transition temperature, is observed in dPS with thicknesses d < 35 nm. For trilayers, a decrease of a,is observed at larger film thicknesses, d < 60 nm. Replacing the fPI substrate with an SiOx substrate, alpha (r) remains bulklike, showing that the length scale for deviations from bulk behavior is sensitive to the specific confining boundaries.