Using the pulse radiolysis technique the OH adducts of cytosine, 5-methyl cytosine and 2'-deoxycytidine were investigated in alkaline N 2 O saturated aqueous solutions. Ab-sorption spectra were recorded and their change with time was correlated with the change in conductivity. An OH -induced reaction (1.4·10 8 )·mol -1 ·s -1 ) was observed with OH adducts of cytosine and 5-methyl cytosine leading to radicals (pK=10.3), the G-values of which were 3.2 and 1.6 respectively (conductivity measurements). These intermediates were assigned to the C-5 OH radical adducts. After completion of the free radical reactions (2k = 6 · 10 8 )·mol -1 ·s -1 ) the resulting product mixture showed pK-values of 8.3 (cytosine, G=0.6), 10.5 (5-methyl cytosine, G=0.8), 10.7 (cytosine, G=1.5; 2′-deoxycytidine, G=1.5) and 12.4 (cytosine, no G-value, determinable with optical detection). Ammonia, which is a product measured after γ-radiolysis of all the three compounds investigated (G(NH 3 ) = 0.6 ± 0.1), is not released within 15 ms after the pulse.