Perspectives of a new approach for the synchrotron photoemission spectroscopic analysis of chemical processes at solid/liquid interfaces under UHV conditions have been explored. A thin layer of HCl-2-propanol solution was frozen-in on the semiconductor GaAs(100) wafer surface by cooling the substrate to liquid nitrogen temperature after etching off the native oxide layer under N2 atmosphere. Chemical reactions induced in situ by exposure to synchrotron radiation (SR) and by stepwise heating have been monitored. Right after etching and freezing, the surface is covered by gallium chlorides with 1, 2, 3, and 4 Cl ions attached and lattice back-bonded to As atoms, as well as by elemental arsenic As0 and 2-propanol. Exposure to SR at low temperature produces surface As chlorides at the expense of As0. The GaCl3 and GaCl2 emissions diminish while GaCl is enhanced. On the other hand, heating the sample to approximately 130 K just above H2O desorption causes the thermodynamically expected reaction of AsCl3 with the substrate GaAs to form Ga chloride species and As0. Heating the sample to room temperature leaves only As0 on the surface and for gallium the content of all surface chlorides is drastically reduced. By further heating to 400 K elemental arsenic starts to desorb and the Ga chloride surface content is reduced. Using different excitation energies the depth composition of the reaction products has been monitored indicating a tendency of decreasing chlorination numbers and increasing Ga vs As chloride content toward the pristine substrate at each stage of the reaction.