Laser flash photolysis of 1,1-diphenyl- and 1,1,3,3-tetraphenylacetone adsorbed onto a series of clays has been carried out under dry conditions. The photochemical behavior of the guests was investigated by using time-resolved diffuse reflectance techniques. Deposition of the organic compounds onto the solids was performed by slow removal of the solvent from dilute solutions at low temperature. Distinct transient spectra in the microsecond time domain were obtained for the guest-host composites. The formation of the diphenylmethyl radical (DPM(.)) was observed with montmorillonites SWy-1 (lambda max = 310 nm) and STx-1 (lambda(max) = 340 nm). The decay of DPM(.) on the bidimensional geometry of the layered clays was found to be much slower than within a tridimensional large-pore zeolite. By contrast, in the case of the pillared Al(13)SWy-1 and Al(13)STx-1 clays, additional reflectance at 450 nm characteristic of the diphenylmethyl cation (DPM(+)) was present. This intermediate presumably arises from the cooperative interaction with the acid sites of the clay. These results show that these ketones are convenient probes to report on the chemical and topological properties of solid surfaces.