Double modulated beam mass spectrometry was used to obtain the first measurements of the identity, relative flux, and energy of film precursors in reactive dc magnetron sputtering. In these experiments, a 2″ diameter silicon target was sputtered in an argon plus deuterium plasma at an argon pressure of 1.5 mTorr. This system produces high quality a-Si:D. Energetic neutral Si and SiD were observed, while energetic neutral SiD2 and SiD3 were absent. The arrival rate of D in the form of SiD increased and then saturated at 19 percent of the total flux as the deuterium pressure was varied from 0 to 5 mTorr. The sputtered Si energy agreed qualitatively with the standard Thompson theory. Energetic ions located at mass 2 and 4 were measured with energies much greater than the sputtered Si. Thermal silane was also detected, raising the possibility that thermal silane radicals in addition to energetic Si and SiD contribute to film growth.