We report the growth of Si1-yCy and Si1-x-yGexCy alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si1-yCy and Si/Si1-x-yGexCy multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of similar to1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si(1-x-y)Ge(x)Q(y) layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.