Passive chemical‐vapor‐deposited diamond layers have the potential to improve thermal conduction in electronic microstructures because of their high thermal conductivities. The thermal resistances for conduction normal to the boundaries of diamond layers, which must be small in order to realize this potential, have not been measured. This research develops two independent experimental methods that measure the total thermal resistance for conduction normal to diamond layers thinner than 5 &mgr;m on silicon substrates, yielding an upper bound for the thermal resistance of the diamond‐silicon boundary. The data for layers as thin as 0.2 &mgr;m agree with predictions that account for phonon scattering on layer boundaries and couple the local scattering rate in the diamond to the grain size. The agreement lends support to the conclusion that the effective diamond‐silicon boundary resistance is dominated by a highly localized volume resistance in the diamond near the interface. ©1995 American Institute of Physics.