We have investigated beryllium metal as a substrate for the microwave plasma chemical vapor deposition of diamond films. The dependence of oxide and carbide interfacial phase formation with temperature and their influence on diamond nucleation and growth behavior were studied by thin film X-ray diffraction. Although a native oxide (BeO with the hexagonal wurtzite structure) remains for the range of substrate temperatures studied (700-800°C), the formation of a carbide (Be2C with the cubic antifluorite structure) is found only above a critical substrate temperature of approximately 750°C. Without the formation of Be2C, the diamond growth rate is low and a significant amorphous carbon component is observed. Just above the critical temperature, films exhibited high growth rates with high phase-purity diamond. Thick films (30 μm) grown above the critical temperature were observed to fracture completely within the Be2C layer, suggesting this to be the weak structural link. © 2000 Elsevier Science S.A.