Decomposition efficiencies η of SiH₄ and the related gas M in SiH₄-M-Ar glow-discharge plasma sustained by Ar carrier gas for fabrication of hydrogenated amorphous-silicon alloy (a-SiX:H) have been measured by mass spectrometry, where gas M is Si₂H₆, SiF₄, GeH₄, C₃H₈, C₂H₆, C₂H₄, C₂H₂, CH₄, CF₄, NH₃, or CO₂. Relative rate constant of dissociation reaction of gas M to SiH₄, K_d,M/K_d,SiH4 determined from η_M and η_SiH4 enables to correlate with the lowest thermodynamic dissociation energy ΔH of the gas M by K_d,M ∝ exp(-ΔH/1.10). Relative in corporation probability into a-SiX:H alloy of relevant radical R for the gas M to SiH_n (n = 0-3) radicals for SiH₄,γ_R/γ_SiHn has been estimated using the film-compositions [X] and [Si] in the a-SiX:H alloy and decomposition efficiencies η_M and η_SiHn in the plasma, where a-SiX:H alloy is a-SiGe:H, a-SiN:H, a-SiC:H, or a-SiO:H. The variation in γ_R/γ_SiHn is at most three times, while that in K_d,M/K_d,SiH4is of two orders of magnitude.