Diamond anvils with diamond encapsulated thin-film microcircuits have been fabricated for ultrahigh pressure electrical conductivity experiments. The diamond films were homoepitaxially deposited onto the diamond anvil substrates with microwave plasma chemical vapor deposition using a 2% methane in hydrogen gas mixture and a diamond substrate temperature of 1300°C. The diamond embedded thin-film microprobes remain functional to megabar pressures. We have applied this technology to the study of the pressure-induced metallization of KI under pressures up to 1.8 Mbar. This technology has the potential of greatly advancing the pressure range of a number of existing high-pressure diagnostic techniques, and for expanding the capabilities of diamond anvil cells into new directions. © 2000 American Institute of Physics.