  Condensation of circumstellar dust begins with formation of molecular
clusters close to the stellar photosphere. These clusters are predicted to act
as condensation cores at lower temperatures and allow efficient dust formation
farther away from the star. Recent observations of metal oxides, such as AlO,
AlOH, TiO, and TiO$_2$, whose emission can be traced at high angular
resolutions with ALMA, have allowed first observational studies of the
condensation process in oxygen-rich stars. We are now in the era when depletion
of gas-phase species into dust can be observed directly. I review the most
recent observations that allow us to identify gas species involved in the
formation of inorganic dust of AGB stars and red supergiants. I also discuss
challenges we face in interpreting the observations, especially those related
to non-equilibrium gas excitation and the high complexity of stellar
atmospheres in the dust-formation zone.
