Experiments were carried out to determine the flow stability of slot-rectangular spouted beds of 300 x 100 mm cross-section with slots of different widths, lengths, and depths. The effects of slot expansion angle and diverging base were also investigated. Dependent variables included the minimum spouting velocity, bed pressure drop and standard deviation of pressure fluctuations. Based on the flow regimes and spout termination mechanisms, instability was found to be mainly due to the interaction of multiple spouts. Criteria were identified relating stable spouting to slot dimensions and particle size. Slots of limited cross-sectional area can provide sustainable and symmetric spouting with little fluctuation, as long as the slot length-to-width ratio and depth are within certain limits, related to those for conventional spouted beds.