Authigenic illite samples have been isolated from preserved reservoir core and characterized by XRD, XRF, SEM and ATEM. Reactivity towards aqueous solutions containing acids, alkalis, complexing agents and reducing agents has been evaluated using both static (flasks) and dynamic (flow-cell) experiments at 80°C and atmospheric pressure. In flask experiments with dilute reagents, reaction appears to be simple stoichiometric dissolution. The same pattern extends to higher concentrations (molar), except for alkalis where Si is preferentially leached leaving an Al-rich residual phase. Under the more vigorous conditions of flow (continuous leaching by fresh reactant solutions), stronger acid and alkaline solutions both cause substantial dissolution although by different mechanisms. Acids displace K+ and cause illite to swell. In many of these experiments, gels were seen to precipitate downstream from the reaction site. These reactions are rapid, taking place in hours at 80°C. Similar reactions can be expected to take place in consequence of a range of drilling, stimulation and production procedures. Clay dissolution, modification and, possibly, migration would be anticipated close to the well-bore and damage from scaling (gels) somewhat further away.