A 820 micron thick single crystal diamond layer epitaxially grown on a single crystal diamond seed (high pressure, high temperature grown synthetic) by microwave plasma chemical vapor deposition with added nitrogen is characterized by an array of analytical techniques before and after annealing the material at high pressures and temperatures. The most striking result is the conversion of the initially dark colored, highly absorbing CVD layer to clear, transparent material after a 1 hour anneal at 7 GPa and 2200 degreesC. IR absorption in the region of the CH stretching modes, 2800 to 3107 cm(-1) shows a remarkable sharpening and persistence of the observed modes. IR absorption in the one-phonon region also indicates the presence of significant concentrations of ionized single substitutional nitrogen in the as grown material. EPR indicates a concentration of neutral single substitutional nitrogen at lattice sites of ca. I ppm, and this changes by less then 30% when annealed at temperatures up to 2200 degreesC. EPR also detects 0.1 ppm of the negatively charged nitrogen-vacancy-hydrogen complex in the as grown diamond, but this anneals out by 1900 degreesC, the negatively charged nitrogen-vacancy complex is below the EPR detection limit in these samples of about 0.1 ppm. Photoluminescence detects the presence of neutral and negatively charged nitrogen-vacancy complexes in the as grown material, and the formation of new, unassigned bands principally in the 800 to 900 nm region. The total detected nitrogen concentration in the sample is ca. 1.5 ppm. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.