Amorphous hydrogenated silicon (a-Si:H) films were deposited at 300 K by the ion-beam-deposition (IBD) method on a Pt(lll) single crystal and characterized by means of Auger electron spectroscopy (AES), electronic and vibrational electron energy loss spectroscopy (EELS, HREELS), and thermal desorption spectroscopy (TDS). The a-Si:H films appear to grow in a two-dimensional mode, and exhibit a polymer-like structure with a hydrogen content of approximately 45 at. %, bonded in monohydride (SiH) and dihydride (SiH2) groups. The films are stable up to 500 K; above this temperature the evolution of hydrogen and the formation of platinum silicides were observed. Exposure to atomic hydrogen leads to the formation of silane, disilane, and higher silicon hydrides. Between 100 and 300 K a constant etching rate of Si/H = similar to0.01 was observed. At higher temperatures the etching rate decreased due to the beginning instability of higher hydrides and the competing process of silicide formation.