Degradation of paper by iron gall inks (IGI) is a challenging issue for written heritage conservation. These inks consist of a mixture of iron(II) sulphate, tannins and gum Arabic, and are therefore acidic and iron-rich. Hence, paper degradation by IGI is often attributed to a combination of acid hydrolysis and of iron-catalyzed oxidation involving hydroxyl radicals (HO●) formed by Fenton reaction. Nevertheless, which of these two mechanisms prevails on cellulose depolymerisation remains a largely open question, which is addressed in the present work. The first step was to look at reactive oxygen species (ROS), especially HO●, by trapping reactions coupled with ESR and HPLC. Traces of HO● were identified on some samples, but their presence was not consistent with paper damage. Another type of ROS was detected in higher quantity, but correlated more to oxidation of iron leached species rather than to paper decay itself. The determination of depolymerisation kinetics at different temperatures led, on inked papers, to activation energies only slightly below those obtained on acidic papers, suggesting a dominant acid hydrolysis mechanism with a limited catalytic effect of iron. This is also supported by the detailed study of the respective effects of iron, oxygen and pH towards degradation of cellobiose taken as a model molecule. This approach gave evidence that acidification of the solution during iron oxidation is the driving force for osidic bond cleavage. To reach this pH, localized iron spots have to be present in the paper as was indeed confirmed by the STXM nano-imaging technique that allowed mapping the distribution of ink components and gelatin within a paper fiber.