We report on the effective cross-plane thermal conductivity of single-crystal Ge layers from 42 to 100 nm on a SiO2/Si substrate in the temperature range 30-300 K. We observe a drastic reduction of the thermal conductivity compared to bulk germanium. A large contribution to the temperature rise in the Ge layer is due to the interfacial thermal resistance between c-Ge/SiO2. The use of a size-dependent intrinsic thermal conductivity of the Ge layer instead of the bulk thermal conductivity improves the consistency with values of the thermal boundary resistance derived from the diffusive mismatch model. Ultrathin films of Ge suffer from a lower reduction of the thermal conductivity compared to ultrathin films of Si, which makes germanium-on-insulator structures promising candidates for devices with reduced self-heating effects compared to silicon-on-insulator structures. (C) 2011 Elsevier Ltd. All rights reserved.