Low-pressure microwave plasma nucleation and deposition of diamond films were investigated in the pressure range 10 mTorr to 10 Torr, at low substrate temperatures of 400-750 °C, using high methane concentration of 2%-15% and oxygen concentrations of 2%-10% in hydrogen plasmas. Diamond film deposition was performed in a microwave plasma system consisting of a microwave plasma chamber, a downstream deposition chamber and an r.f. induction heated sample stage. The deposition system can be operated in either localized microwave plasmas or extended electron cyclotron resonance (ECR) plasmas by varying the sample stage position. Initial nucleation for diamond growth was achieved by ECR plasmas under a low pressure of 13 mTorr and at 600 °C. Diamond nucleation on BN and SiC interlayer materials was demonstrated by depositing well faceted diamond crystallites onto amorphous BN and cubic SiC coated silicon substrates. Low-pressure ECR microwave plasma deposition and the use of BN and SiC as interlayer materials for coating substrates are new practical methods for generating diamond nuclei to promote diamond film growth on non-diamond surfaces. Cathodoluminescence (CL) studies on diamond films deposited at 10 Torr pressure show that CL emissions at 430, 530, 560 and 740 nm can be employed to characterize the quality of diamond films. Scanning electron microscopy (SEM) of diamond films deposited at 600 °C with 5% CH4 and 5% O2 in H2 plasmas has shown high-quality well faceted crystallites of 1 2 μm sizes. CL measurements on these diamond films show very few nitrogen impurities and no detectable silicon impurities.