Abstract The electron spin resonance spectra have been measured for the radical formed by the photolysis of methylene iodide adsorbed on silica gel. The spectra were anisotropic and were attributed to the radical of the ≡Si–O–CH2form by an analysis of the hyperfine splittings due to two protons, taking into consideration the photochemical processes on the silica gel surface. From a line-shape analysis of the observed spectrum, the following anisotropic hyperfine interaction parameters for the protons and g-value have been obtained: A_//=28.5±0.5G, A_⊥=18.8 ±0.5G, g=2.0030±0.0005. It has been concluded from the analysis of the spectra that the hyperfine structure observed with axially-symmetric tensors is a result of the fast internal rotation of the methylene group of the radical.