The superb mechanical and electrical properties of diamond make it an attractive material for use in extreme conditions. Diamond devices have been fabricated. but the combination of diamond with other materials to form alloys is not yet well-understood. We have investigated the electrical and optical properties of diamond implanted with Si, which is in principle isoelectronic in diamond. The diamond layers were 23 mum thick p-type layers grown on Si(1 0 0) substrates. Silicon was implanted at room temperature, at energy of 300 keV with doses of up to 8.0 x 10(16) cm(-3). A two-stage post-implant anneal process was then performed at 500 degreesC for 30 min, then 800 degreesC for 30 min. Rutherford backscattering spectrometry (RBS) measurements indicated Si concentrations up to 2 at.%. in agreement with simulations of implant profiles. X-ray measurements indicated that about 60-70% of the Si was incorporated substitutionally. corresponding to up to 1.5 at.% from linear interpolation of lattice constants. Raman spectroscopy measurements confirmed that the diamond structure was reconstructed with the post-implant anneal treatment. NEXAFS measurements confirmed the reconstruction of the sp(3) diamond bonds from sp(2) graphite after annealing. Electrical measurements indicated an increase in diode current density with increased Si dose and a decrease in contact resistance with contact anneals. These novel diamond alloys may be useful for electronic and optical devices. We report on the preparation and properties of these alloys.