Ultralow dielectric constants have been achieved for thin porous films of high-T-g polymers. Rapid solid-state polymer expansion applying supercritical carbon dioxide was used to create nanosized cavities in thin films of aromatic polyimides (see Figure). Foamed films with spherical voids less than 10 nm in size as well as multilayered microstructures with interlayer distances of less than 20 nm were made with k values as low as 1.77.