AbstractWith a view to exploring the possibility of eliminating the explosion hazard in the manufacture of sodium azide, the limits of inflammability were de ermined in the upward, horizontal and downward directions for all mixtures of ammonia, nitrous oxide and air at normal Temperature and pressure in 2 in, diameter rubes. The range of inflammability was found to be greatest in mixtures containing no air, being 2·6‐71·9% of ammonia for upward propagation, 5·1‐71·6% for horizontal propagation and 6·8‐71·0% for downward propagation. Addition of air caused contraction of the inflammable range at both ends roughly in proportion to the amount of air added. The limits for mixtures of ammonia and nitrous oxide were also determined (a) in 3 in, diameter tubes to indicate the general effect of cross‐sectional area and (b) at 150° and 250° to show the effect of temperature. An increase in either the tube diameter or the temperature produced a slight widening of the limits. Mixtures of ammonia, nitrous oxide and air appeared to obey Le Chatelier's limit rule except for some discrepancy affecting lower limit mixtures. Other evidence confirmed the abnormal behaviour of nitrous oxide containing small amounts of ammonia. AS these mixtures were liable to explode violently, although apparently below the limit for normal flame propagation, their anomalous behaviour is attributed to the exothermic decomposition of nitrous oxide, probably catalysed by the presence of small amounts of ammonia. This reaction appeared to propagate by a mechanism more akin to detonation th