The development of A.N. Terenin's studies of the "active action of light" on heterogeneous gassolid systems is considered. Photoactivated adsorption, desorption, and chemical reactions are analyzed in stages: photon absorption by a solid → excitation energy transfer and dissipation → center activation → disruption (formation) of linkages between molecules and activated centers. Experimental studies are carried out in situ in three phases: gaseous, adsorbed, and solid. Original techniques, such as photo-mass spectrometry, thermal desorption spectroscopy, optical and luminescent spectroscopy, and UV photoelectron spectroscopy, are used.