The gas flow in the freeboard of a fluidised bed is strongly dependent on bubble eruption at the bed surface. Here, the gas how above an erupting bubble has bean studied by the injection of single bubbles in an incipiently fluidised bed. Gas velocity vector maps of the vertical central plane above the bed surface, i.e. a plane parallel to the direction of the overall flow, have been determined using image shifting (an implementation of particle image velocimetry). These vector maps are presented and discussed. A mechanism has been proposed to describe the process of single-bubble eruption.