We show by ex-situ STM that pit-free H-Si(111) surfaces can be prepared by immersion of pre-oxidized Si(111) in deoxygenated 40% NH4F for 15 minutes. In contrast, H-Si(111) prepared in air-saturated 40% NH4F for 15 minutes results in small etch pits randomly covering ca. 1% of the surface. Added dioxygen, shorter etching times or stirring of the air-saturated etch solution increase the etch pit density. Dissolved oxygen is responsible for the initiation of etch pits on the (111) monohydride terraces. The reaction between dissolved oxygen and the H-Si bond of the (111) terrace is limited by mass transport of dissolved oxygen to the silicon surface. A proposed reaction mechanism of dissolved oxygen with H-Si is briefly mentioned, which explains not only etch pit initiation in ammonium fluoride but also the oxidation of hydrogenterminated silicon in wet, oxygenated environments. The results presented here for H-Si(111) lead to a greater understanding of the wet oxidation and etching processes of this important model surface.