Abstract Experimental study on the flow patterns and transitions in a rectangular spout–fluid bed (cross-section of 300 mm × 30 mm and height of 2000 mm) packed with Geldart group D particles was carried out. Six distinct flow patterns, i.e. internal jet, spouting, fluidizing, jet in fluidized bed with bubbling, jet in fluidized bed with slugging and spout–fluidizing were identified. Criteria as well as schematic diagrams and typical flow pattern images obtained by a high-resolution digital CCD camera were presented for classifying these flow patterns. Typical flow pattern maps were plotted for describing the transitions between flow patterns with operating conditions. In addition, two kinds of flow instabilities were observed. Based on the flow pattern images recorded by the digital CCD camera, the mechanism of flow instabilities was preliminarily discussed. It was found that the flow instabilities are caused by stochastic visible large bubbles in the boundary of spout jet and near the fluidizing gas distributor. The growth of surface disturbances might exacerbate these flow instabilities.