Ultra-fast magnetic resonance imaging (MRI) has been applied for the first time to study the rise velocities of bubbles and slugs in a 3-D gas-fluidised bed. Ultra-fast MRI is a non-intrusive technique, providing measurements with both excellent temporal and spatial resolutions. MRI measurements reveal that the excess gas velocity, U -U-mf, does not affect the rise velocity of continuously formed bubbles, but does influence the rise velocity of continuously formed slugs. With regard to the rise of bubbles and slugs, a slugging gas-fluidised bed can be divided into three regions. In Region I, near the distributor, the rise velocity of bubbles is significantly influenced by coalescence. In Region II, higher up, an almost constant velocity of about 0.38 x root gD(t) is observed. Far above the distributor, the excess gas velocity must be added to the rise velocity of a single slug, giving U-S = (U - U-mf) + 0.35 x root gD(t). When two bubbles coalesce, they accelerate, but the single bubble arising from their coalescence moves at first more slowly than its final velocity, which is reached after a short period (0.3-0.4s) of post-coalescence acceleration, associated with a change of shape: in fact, the bubble formed by coalescence has an irregular shape and has to deform to reach its final velocity. (c) 2006 Elsevier Ltd. All rights reserved.