A petrographic and isotopic analysis of the carbonate target rocks at the Jeptha Knob structure, the Serpent Mound impact structure, and the Decaturville impact structure was conducted to better understand potentially impact-related diagenesis of carbonates. Impact craters form during catastrophic events when objects from space hit the Earth’s surface at high velocities and have high kinetic energy which is then partially transformed to thermal energy and leads the target rocks to experience high pressures and temperatures that deform them and can result in the initiation of a hydrothermal system (e.g.,Osinski et al., 2012).The goal of this study was to further the understanding of hydrothermal alteration related to the three relatively small complex impact craters in carbonate rocks, and to discuss the issues in distinguishing impact related diagenesis from other diagenetic events. The target rocks at two complex impact structures, Serpent Mound (Ohio), Decaturville (Missouri) and one probable impact, Jeptha Knob (Kentucky) were petrographically characterized regarding authigenic and hydrothermal phases present. Paragenetic sequences were developed for all three structures and authigenic phases include pyrite, marcasite, hematite, magnetite, dedolomite, quartz, baroque dolomite, K-feldspar, biotite, and clays. Of these authigenic phases a number could be associated with hydrothermal deposits including marcasite, quartz, K-feldspar, biotite, clays, and baroque dolomite which have been reported as being common in post-impact hydrothermal systems (Osinski et al., 2012). Other evidence of hydrothermal activity, while not pervasive, includes veins filled with calcite and pyrite in some samples at Serpent Mound and Decaturville. It is difficult to distinguish between impact related hydrothermal fluids and other diagenetic events (e.g. orogenic fluids). Many of the authigenic minerals are localized to the impact structures suggesting they could be related to an impact-generated hydrothermal system, although the impact process could have produced fluid conduits for later diagenetic events. The results of this study suggest that hydrothermal alteration at small complex impact craters in carbonate target rocks can be identified but the alteration can be hard to distinguish from other diagenetic events.