Electromagnetic interference (EMI) shielding performance of chemically and thermally reduced graphene aerogels (GAs) was systematically studied. The EMI shielding mechanisms were extensively analyzed in terms of the distinct surface characteristics resulted from the different reduction methods for the first time. EMI shielding effectiveness (SE) of chemically and thermally reduced GAs reached 27.6 (GAC) and 40.2 dB (GAT) at the thickness of 2.5 mm, respectively. It was found that the introduction of nitrogen atoms through chemical reduction induced localized charges on the carbon backbone leading to strong polarization effects of GAC. The relatively incomplete reduction caused a large number of side polar groups which prevented the graphene sheets from pi-pi stacking. In contrast, the higher extent of reduction of graphene sheets in GAT left a smaller amount of side polar groups and formed more sp(2) graphitic lattice, both factors favored pi-pi stacking between the adjacent graphene sheets, resulting in higher electrical conductivity and enhanced EMI SE. The EMI shielding performance of the GAs prepared outperformed the recent reported porous carbon materials with respect to the absolute SE value at the similar thickness and/or density. (C) 2017 Elsevier Inc. All rights reserved.