The importance of reactions involving CO2, brine, and rock:formations into which CO2 is injected for CO2 sequestration in saline aquifers is understood. However, the pore-level changes that occur due to these reactions under flow conditions-and their impact on the ultimate fate of CO2 in the repository have not received the same level of attention because of the perceived slowness of-the carbonation reactions. In this paper we-examine these reactive changes and their impact on the pore structure in sandstones and limestones at realistic aquifer pressure and temperatures, and under reactive flow conditions. The-changes observed at the pore-level by direct porosity and microcomputer tomography measurements were complemented by the measurements of time-dependent effluent concentrations of target cations. It is observed that iron chemistry plays an important role in the dissolution and precipitation reactions in Berea sandstone. Elite dissolution leads to a peak in iron concentration in effluent brines. A higher level of dissolution and an increased porosity ate observed near the inlet of the cote. In limestones, consistent dissolution is observed throughout the experiment Wormholes are also generated for experiments with larger total flow rates. Results show that reactive changes can cause significant pore level changes over a! short injection span during CO2 sequestration in saline aquifers with profound implications on injectivity and possibly major mechanical changes.