The reactions of SO₄˙⁻ with 2′-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO₄˙⁻ radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H˙ atom). In the reaction of SO₄˙⁻ with 1a (k = 1.6 × 10⁹ dm³ mol⁻¹ s⁻¹), a transient (λ_max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (λ_max ≈ 385 nm) does not react with O₂ and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O₂, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 × 10⁻⁷ mol J⁻¹ upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at λ_max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 × 10⁴ s⁻¹). In the presence of O₂, the decay is much faster (k ≈ 1.3 × 10⁹ dm³ mol⁻¹ s⁻¹) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2′)-OH group with the cytidine SO₄˙⁻-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 × 10⁻⁷ mol J⁻¹ (85% of primary SO₄˙⁻) at high dose rates (electron beam irradiation). At low dose rates (γ-radiolysis) its yield is increased to 7 × 10⁻⁷ mol J⁻¹ due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO₄˙⁻-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO₄˙⁻ (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.