We use infrared absorption spectroscopy in the multiple internal reflection geometry to investigate in sills and in real-time the chemical process at Si(100) surfaces that are immersed in a dilute solution of ammonium fluoride, NH4F. We have followed spectral changes in the Si-H stretch, vibration region of the surface during immersion in the solution. Infrared spectroscopic data show that the native oxide on the surface is initially removed by NH4F solution and subsequently, H-terminated Si(111) microfacets are increasingly populated. Long-term immersion in dilute NH4F leads to oxide formation on the surface. The rate of oxide formation strongly depends on the concentration of NH4F; oxide formation is enhanced at concentrations below 10% and it does not occur at concentrations above 20%. We suggest that oxide formation and subsequent oxide removal proceed on Si surfaces in dilute NH4F solution and that at low NH4F concentration, the rate of oxide formation is higher than that of oxide removal.