Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of controlling flammable hydrogen air mixtures by modifying the flammability limits to prevent ignition. A simplified theoretical model, based upon the inhibition parameter concept, is developed allowing the effect of nitrogen dilution and very fine water fog upon the upper flammability limit (UFL) of hydrogen-oxygen-nitrogen mixtures to be predicted and compared with corresponding experimental test data. The model predicts similar general trends in behaviour with regard to the effect of water fog upon the UFL to those observed in the experimental tests. The results suggest that the combination of nitrogen dilution and high density water fog can produce a significant reduction in the range of the UFL and total extent of the flammable region. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.