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New types of bulk SiOC glass matrix composites reinforced with diamond particles were successfully fabricated. Diamond particles were introduced into the SiOC matrix by the polymer-derived ceramics (PDC) route using polysiloxane and two different sized diamond particles, 2 and 30 μm as starting precursors. Dense bulk specimens were prepared by warm pressing at 150°C–160°C followed by pyrolization at 1100°C for 2 h. The composites were characterized by means of SEM, TEM, optical observation, X-ray diffraction, and Vickers indentation. Elastic properties were determined by means of ultrasonic echography, and Young's modulus was found to increase from 96 to 154 GPa when diamond content increased from 0 to 25 vol%. Reinforcement results in significant improvement, nearly 100%, in hardness compared to pristine SiOC glass sample. The size of the diamond particles has an influence on density and microstructure of the composites. The TEM investigations reveal that SiOC glass matrix and 2 μm diamond particles have excellent bonding. The present study demonstrates the possibility of fabricating bulk SiOC glass–diamond composites via the polymer-processing route, resulting in composites with promising mechanical properties.