The compounds [meso-tetrakis(1-methyl-4-pyridiniumyl)porphyrinato]iron(III ) (FeIIITMPyP) and [meso-tetrakis(3,5-dichloro-1-methyl-4-pyridiniumyl)porphyrinat o]iron(III) (FeIIITCl2MPyP) photocatalysed the release of bases from nucleosides and their derivatives. The ribonucleosides, of which cytidine (C) gave the highest yield, produced much higher yields of free base than the corresponding deoxyribonucleosides. Under an argon atmosphere, virtually quantitative reduction of FeIIITMPyP into FeIITMPyP by C or 2'-deoxycytidine (dC) was observed and the reduction by C was much more effective than by dC. The increased reactivity of ribonucleosides relative to deoxyribonucleosides was ascribed to a difference in the binding properties of the porphyrin-nucleoside interactions and to base-releasing degradation of ribonucleosides from their C-2' carbon radical.