The propagation of edge localized modes (ELMs) towards the main chamber could lead to potentially serious consequences in ITER (Federici G et al2001 Nucl. Fusion41 1967). This effect was studied in JET by examining the interaction of a large sample (>200) of Type-I ELMs with an array of Langmuir probes on the outboard limiter. High clearance discharges were used with a separatrix to wall distance of 8–18 cm. The average ELM duration was measured as ∼ 200 ± 50 µs, the average radial propagation velocity of the ELM plasmoid in the scrape-off layer as ⟨v⊥⟩ ∼ 0.45 ± 0.1 km s−1 or as a fraction of the sound speed, ⟨v⊥/cs⟩ ∼ 0.2%, and its diffusion coefficient, D⊥, as 500 ± 100 m2 s−1—a significant increase over inter-ELM levels. The above velocities are in good agreement with the lower (∥ loss) limit of the sheath-limited model of plasmoid propagation (Krasheninnikov S I 2001 Phys. Lett. A 283 368, Krasheninnikov S I et al 2002 29th EPS Conf. on Controlled Fusion and Plasma Physics (Montreux, Switzerland)).