Spectral methods can provide a variety of possibilities to determine several types of radiation-induced DNA damage, such as nucleobase destruction and local denaturation. DNA UV absorption and CD spectra measured at room temperature undergo noticeable alteration under the action of γ-radiation. We have applied the Spirin method of total nucleobases determination, and have measured the molar extinction coefficient of DNA and DNA CD spectra for solutions with different NaCl concentrations (3 mM–3.2 M) and containing MgCl2, exposed to γ-radiation with the doses of 0–103 Gy. The melting temperatures of DNA in irradiated solutions at the doses of 0–50 Gy were obtained with the help of spectrophotometric melting. It was found that the amount of destructed nucleobases and radiation-induced loss of DNA helicity significantly decreases with the rise of the ionic strength of the irradiated solution. Substitution of a portion of Na+ ions on Mg2+ while keeping the total ionic strength constant (μ = 5 mM) does not affect the considered radiation effects. The role of the structure and composition of the DNA secondary hydration layer in the radiation-induced damages is discussed.