Hydrogenated defects in microwave plasma chemical‐vapor‐deposited (CVD) diamond films have been examined by proton nuclear magnetic resonance and infrared spectroscopy and correlated with the thermal conductivity of these samples. The measured thermal conductivities of 1–22 W cm−1 K−1are consistent with an upper limit predicted based on the assumption that the scattering rate from hydrogenated defects is proportional to the as‐deposited hydrogen contents of ≤0.5 at. % H. This hydrogen is predominantly clustered, consistent with hydrogen passivated diamond surfaces. Thus, the as‐deposited hydrogen content may be a measure of the internal surface area of the polycrystalline diamond film which is hydrogen passivated. Part of this surface area may be due to elongated voids, several microns in dimension, which are observed by scanning electron microscopy of polished diamond film cross sections. These voids may arise from the columnar film growth process and thus be a common feature of CVD diamond films. Phonon scattering from these voids which are oriented along the film growth direction could account for film‐to‐film variation in and the anisotropy of the thermal conductivity of CVD diamond films. ©1995 American Institute of Physics.