The fabrication of Integrated-Circuits requires many different steps, including growth or etching. Since device performance strongly depends on surface preparation and control at each step, we have developed in situ Fourier transform infrared (FTIR) spectroscopy using multiple internal reflections to monitor the nature of interfaces. At the heart of high speed silicon technology is the growth of ultra thin oxide layer on top of the Si substrate. We monitor here the structure of such thermally grown oxides by sequential etching in dilute HF. The analysis of the LO and TO vibrational phonon modes of the oxide at 1070 and 1270 cm@super -1@ respectively shows that the etching mechanism exhibits two kinetic regimes depending on whether the HF flow wets the surface in a static or dynamic way. For static wetting, the LO absorption of the oxide undergoes a dramatic distortion that can be related to the unusual nature of the diffuse layer in the vicinity of the oxide surface. The etching mechanism depends in effect on the competition between diffusion and kinetics in this layer. In contrast to silicon, the passivating oxides of InP substrates are rather intricate. We examine here InP wafers that are covered by a thin oxide typical of "epi-ready" wafers offered by vendors. Analysis of the FTIR spectra shows a coexistence of several phases, such In@sub 2@O@sub 3@ (850 cm@super -1@) and InPO@sub 4@ at higher frequencies along with mixed oxide phases at intermediate frequencies. These oxides can also be removed by etching in HCl (10 wt%). This leaves a clean but very reactive InP surface that quickly attracts contaminants so that in situ analysis is highly desirable. In addition to the in-situ spectroscopy, we have devised an electrochemical control of the surface that makes it possible to further modify the composition of the adlayer as well as the diffuse layer.