The empty states of graphene and graphene adsorbed on nickel (1 1 1) are studied using a plane-wave pseudo-potential method based on local density functional theory. The analysis is used to assign spectroscopic features observed in secondary electron emission experiments either to the graphene sheet or to the nickel substrate. The calculations provide insights into the key role of the graphene–substrate interaction, through the hybridization between carbon and nickel states, and indicate secondary electron emission as an efficient probe for interfaces made of two dimensional crystals.