Stationary and non-stationary etching of Si(100) surfaces by hydrogen were studied between 200 K and 800 K using direct product detection and thermal desorption spectroscopy. Silane was the only etch product observed. The rates of silane SiDnH4-n isotopes measured during etching D-saturated Si(100) surfaces with gaseous H illustrate that the etch reaction proceeds between surface silyl and incoming H in a direct (Eley-Rideal or hot-atom) reaction step: H(g) + SiD3(ad) --> SiD3H(g). Non-stationary etching via silane desorption occurs through disproportionation between surface dihydride and silyl groups, SiH2(ad) + SiH3(ad) --> SiH4(g).