This experimental study focuses on the unsteady pressure characteristics of two-box bridge decks. Models with vertical plates and gratings in several gap length configurations are investigated and compared with single-box cross sections. The results demonstrate that, with respect to unsteady pressure characteristics, the upstream box is barely affected by the slot and behaves similarly to the single-box model. In contrast, the unsteady pressure characteristics of the dowstream box are defined mainly by the characteristics of the slot and the geometry of its cross section. The upstream box geometric configuration plays only a minor role in the definition of the unsteady pressure characteristics of the dowstream box. An equivalent Theodorsen function to be used in the calculation of aerodynamic derivatives of two-box girders is proposed. Using this function, it is demonstrated that the only aerodynamic derivative that is aerodynamically dependent on the gap length is A. These findings suggest that the results of single-box investigations can be extended to two-box cross sections. (C) 2014 Elsevier Ltd. All rights reserved.