A study has been made of the microstructure of rigid closed-cell polyurethane foams in the density range 35 to 420 kg m−3. Existing models for the structure of foams of this type have been evaluated using optical and scanning electron microscope techniques. Foams with a density of the order of 35 kg m−3 are shown to be best represented as a pentagonal dodecahedron. Medium density foams from 70 to 300 kg m−3 have a structure described as rounded polyhedra and high-density foams from 300 to 420 kg m−3 have an isolated spherical structure. A relationship between average cell size and density is given.