Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH4-H-2) gas mixtures with input flows of 50 sccm Ar, 2-20 sccm SiH4 and 0-50 sccm H-2 at total pressures of 1-4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H-2 input, the densities of SiHx (x < 4) radicals show evolution trends depending on the SiH4 and H-2 input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiHx density trends with H-2 input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.