The flammability of structured packing made of thin aluminum sheet (0.2 mm) for use in oxygen service has been investigated by several companies and the results have been published since the first paper was given by Dunbobbin et al. . All these previous tests involved small samples of about 100 g. The propagation on such small samples is quite difficult to analyze especially with LOX flowing down, as in actual operating conditions. The results of these tests have been published in STP 1197 . Three sets of tests have been performed on larger samples to closely reproduce the operating conditions of cryogenic columns. The first set involved larger packing samples (650 g) above a LOX pool, heated to create a gas flow. The second set involved seven complete columns, 30 cm diameter × 2 m high, containing packing with LOX flowing down. The ignition pack was similar to the first set of samples. The third set involved twelve complete columns, 30 cm diameter × 2 m high, with 20 aluminum trays, in normal working conditions. The ignition cell failed in six tests so only six tests were significant. Results of this investigation performed on large samples were used when preparing the CGA pamphlet G-4.8 published in 1993 , which concluded that aluminum structured packing in ASUs introduces no additional risk, compared with aluminum trays.