The matrix effects on trapping and degradation of methylal radical cation generated in solid argon, krypton, and xenon doped with an electron scavenger at 16 K were investigated by EPR spectroscopy. A relatively weak characteristic signal from trapped methylal radical cations was recorded in an argon matrix. However, the most intense signals in this matrix result from the products of degradation of the primary cations, mainly methyl and methoxy radicals. A comparatively low g(max) value for methoxy radical (g = 2.032) was explained by formation of a pi-complex [CH3O.... CH2=O+CH3] upon fragmentation of the parent cation in solid argon. In the case of a xenon matrix, the main observed species is the (CH3OCHOCH3)-C-. radical, which appears to result from proton loss in the primary cation. Both deprotonation and fragmentation products were found in krypton. The matrix effects were attributed to the variations in ionization potentials and polarizability (or basicity) of matrix atoms. Fragmentation predominating in argon was explained by excess energy resulting from highly exothermic positive hole transfer. Deprotonation in xenon is favored by basicity of the matrix atoms. Two possible mechanisms were discussed for the latter case, i.e., thermodynamic effect (deprotonation to matrix) and kinetic effect (catalysis of intramolecular H shift by matrix atoms).