Undoped and carbon-doped magnesium diboride (MgB2) samples were synthesized using two sets of mixtures prepared from the precursors, amorphous nanoboron, and as-received amorphous carbon-doped nanoboron. The microscopic defect structures of carbon-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman and electron paramagnetic resonance spectroscopy. Mg vacancies and C-related dangling-bond active centers could be distinguished, and sp(3)-hybridized carbon radicals were detected. A strong reduction in the critical temperature T-c was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. (C) 2015 AIP Publishing LLC.