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The paper is concerned with the action of ionizing radiation on surface properties of silica-gel. Thermally pretreated silica-gel samples were irradiated in vacuum by gammarays from a Cow source, and investigated by the electron spin resonance (e.s.r.) method.Scission of bonds in s d a c e OH groups of silica-gel, leading to formation of hydrogen atoms stabilized in the surface layer, was shown to take place in samples irradiated at the liquid-nitrogen temperature. Adsorbed hydrogen atoms were highly reactive and interacted chemically with oxygen and ethylem at -150 and -120°C. An ethyl radical spectrum was obswed in the latter case. The hydrogen-atom signal from samples irradiated at the liquid-nitrogen temperature was accompanied by a complex signal caused by silica-gel defects and consisting of at least four different superimposed lines.The formation of Fcentres in samples irradiated at room temperature took place on the surface and in the bulk of silica-gel. Very weak lines of a hyperhe structure, possibly connected with hyperfine splitting due to interaction of odd eleztrons with hydrogen atoms of surface hydroxyl groups were also observed. An asymmetric line probably

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