The explosibility of six industrial dusts has been investigated in a large scale vertical tube apparatus. All six dusts were classified for explosibility in the standard small scale test apparatus. Direct comparison is made between explosibility in both types of apparatus. Flame propagation was obtained in the large scale apparatus with marginal Class I dusts but no flame propagation was obtained in that apparatus with the Class II dusts. The behavior of the dusts in the large scale vertical tube apparatus was similar to that obtained previously with phenol-formaldehyde resin/magnesium oxide mixtures with the same explosibility Classifications.