Diamond is deposited from 100 seem H-2/0.5 seem CH4 or 100 seem H-2/4 seem CH4/2 seem O-2 by microwave plasma assisted chemical vapour deposition, either in the middle of the plasma ball or in a remote position at the level of the bottom wall of the wave guide. When H-2/CH4 mixtures are used, from Raman spectroscopy the parasitic phases concentration and the full width at half maximum (FWHM) of the diamond line are seen to decrease from the middle to the bottom position. In the bottom position, the diamond crystallite characteristics are further improved by using the mixture containing oxygen : no parasitic phases can be detected, and the FWHM remains between 3 and 4 cm(-1) between 680 degrees C and 880 degrees C. The concentration of luminescent centers (from the Raman backgrounds) also decreases for a lower deposition temperature, O-2 in the gas, and a remote substrate position, but has a minor effect on the FWHM. These centers are ascribed to defects in the bulk of the crystallites while the FWHM and non-diamond Raman signal are both ascribed to parasitic phases on the crystallite surfaces. Coherent phenomenological models are proposed for the origins of the decrease of the FWHM and of the concentration of the luminescent centers with the previous deposition conditions.