Low-pressure silane plasmas (2-20 Pa) diluted with the noble gases helium and argon as well as hydrogen were generated by microwave excitation in order to determine plasma parameters and absolute particle number densities. Specific silane radicals (SiH, Si, , H) were measured by means of optical emission spectroscopy, whereas particle densities of silane, disilane and molecular hydrogen were measured with mass spectroscopy. Experimental results confirm model calculations, which were carried out to determine number densities of all silane radicals and of higher silanes as well as electron temperature. The electron temperature varies from 1.5 to 4 eV depending on pressure and gas mixture. The temperature of heavy particles is 450 K and the electron number density is . The rotational temperatures of SiH are between room temperature and 2000 K due to increasing dissociative excitation. In the plasma the number density of silane is reduced, whereas the number density of molecular hydrogen is close to the silane density, which is fed in. Particle densities of , disilane and atomic hydrogen are in the range of a few per cent of the silane number density. At low pressure the density is similar to and decreases with increasing pressure due to heavy particle collisions with silane producing higher silanes. Particle densities of SiH and Si are only in the range of some of the silane density decreasing with increasing collisions of heavy particles with silane and molecular hydrogen. In mixtures with argon Penning reactions increase the silane dissociation.