The incorporation of boron into diamond films during the growth process was studied. The following results were obtained by using a novel boron gaseous source : trimethylboron diluted in helium. Two different deposition techniques were compared : hot filament chemical vapour deposition (CVD) and microwave plasma CVD. In both systems, films were deposited from mixtures of methane and hydrogen and variable amounts of trimethylboron. The effects of boron on the growth process and on the film’s structure were investigated. Wavelength-dispersive electron probe X-ray microanalysis showed that a wide range of boron contents in the films were obtained and that boron concentrations in the films were proportional to the concentration of trimethylboron in the reaction gases. Also, a uniform level of boron through the films was observed by secondary ion mass spectrometry. The boron incorporation into the films was higher in the hot filament reactor deposition than in the microwave system. Electron microscopy, X-ray diffraction and Raman spectroscopy showed that high boron concentrations in the films induce definite morphological and structural effects. The boron doping causes a great increase in the film’s conductivity up to saturation in the highest doping levels.