Compacts of boron-doped synthetic diamond with extremely high doping level are obtained at a pressure of 8–9 GPa and temperature about 2500 K from graphite-boron carbide (5 or 7%) mixtures. The boron content in the diamond estimated by the diamond lattice parameter (0.3573–0.3575 and 0.3576–0.3578 nm, respectively) is about 1–3% and 3–4%, respectively. Thus obtained compacts showed the highest electroactivity of all known diamond, diamond-based, and diamond-like materials (by example of anodic chlorine evolution reaction). In compliance with the earlier found general trend for all known diamond, diamond-based, and diamond-like materials, with the increasing of the compact doping level some increase in the material’s electroactivity also occurs. The heavily boron-doped diamond compacts can be used as indicator electrodes in the electroanalytical determination of ethylenediaminetetraacetic acid in aqueous solutions (by its electrooxidation current).