Ultramicrotomes are generally used for preparation of very thin sections for transmission electron microscopy (TEM). Earlier we showed that instrumented ultramicrotomes can be used for measurement of fracture energy of polymers. Our instrumentation is a sample holder which uses two piezoelectric force transducers measuring two force components simultaneously. The fracture energy was almost in the same order of magnitude as theoretical predictions from chemical bond fracture only (approximately 10J/m exp 2 ) as the section thickness is decreased to around 50 nm. Since the energy for chain scission showed to be such a large part of the fracture energy we suggest that instrumented ultramicrotomy is useful when information on covalent bond density is of interest. Therefore, the method has potential to quantify nanoscale crack growth in polymers and other materials. The chemical structure and cross-links had as expected very little effect on fracture energy at small section thickness.