Point-defect agglomeration, hydrogen loss, mass densification and a grain-boundary localized sp(2)-phase alteration was observed when d.c. are-jet chemical vapor deposition (CVD) diamond was vacuum annealed at 1273 K for 60 min. The size, volume fraction, and spatial distributions of the defects before and after the anneal were assessed using Doppler broadening and lifetime positron annihilation spectroscopy, high-resolution transmission electron microscopy/transmission electron energy loss spectroscopy, and forward recoil spectrometry and indicated that both inter- and intra-crystalline modifications to the defect characteristics occur, with significant structural and chemical alterations spatially located at/near grain boundaries. These observations corroborate the CVD diamond defect "spectrum" inferred by thermal conductivity measurements, modelling, and structural analyses.