Two types of nanosilica-based urea–formaldehyde (UF) hybrid materials with formaldehyde to urea (F/U) ratio (0.8) were synthesized without (UF + SiO₂) and with coumarin (4-chloro-3-nitro-2H-chromen-2-one). Obtained composites have been irradiated (50 kGy) and their radiation stability was evaluated on the basis of thermal behavior before and after irradiation. The thermal properties of materials were studied by nonisothermal thermogravimetric analysis, differential thermal gravimetry, differential thermal analysis and differential scanning calorimetry supported by data from infrared spectroscopy. The free formaldehyde percentage was determined for all prepared samples. The shift of temperature values for selected mass loss to a high temperature indicates the increase in thermal stability of materials with coumarin after -irradiation. The free formaldehyde percentage was reduced after irradiation from 15% to 3% for sample without coumarin and from 7% to 4% for sample with coumarin. The asymmetric stretching vibrations of –N–CH₂–N– absorption band and _as(Si–O) of siloxane or silicone in FTIR spectra of irradiated coumarin compound are shifted to higher wave number values and confirmed its higher thermal stability.