The etching of n‐type silicon (111) has been investigated by means of in situ scanning tunneling microscopy (STM) observations performed over a wide range of bias of the sample. A special procedure has been used to observe topography changes at potentials close and positive of the rest potential. Irrespective of the bias, images show that the surface consists in atomically smooth terraces separated by 3.1 Å high steps. At cathodic bias, the etching occurs principally at terrace edges and (111) terraces are most probably H terminated, which prevents their reconstruction, as could be seen in atomically resolved pictures taken in situ. Triangular etch pits nucleate when the potential approaches the rest potential. The Si‐H coverage is, however, preserved despite the continuous removal of Si atoms from the surface. Beyond the passivation potential, a high density of etch pits is developed on the terraces, although the dissolution rate decreases. It is shown that the etch rate of the dissolution can be derived from sequences of STM images and that it presents a maximum, close to the rest potential, as it has been found previously with long‐term material loss measurements. The present STM results yield new insights into the surface chemistry and the anisotropy of the reaction. The complementary electrochemical characterization of the etching process will be outlined in Part II of this paper (the following article) where a detailed reaction mechanism is presented.