Despite the vast number of carbon-supported catalysts in industrial processes, the relationship between the active catalyst and the carbon framework is not well understood. In this study, we report the importance of the carbon porosity on Cu-catalyzed carbon dioxide reduction (CO2RR) using carbon aerogels, a class of high surface area carbon foams with tunable structural properties. Systematically adjusting the porosity of the carbon aerogels leads to significant differences in catalytic selectivity and activity. We demonstrate that the pore widths of the carbon aerogels are correlated with CO2RR product selectivity, with an optimal mesopore size of 30-50 nm for CO and C2H4 formation. In this size region, the CO2RR activity experiences a 2-fold enhancement, while HER activity remains similar across all tested samples. Given the ubiquity of carbon materials, our findings will inform the design of carbon-supported catalysts for CO2RR and other industrially relevant chemical transformations. (C) 2018 Elsevier Ltd. All rights reserved.