A copy of the standard ASTM spark generator for determination of MIEs of gases and vapours was built and measurements to determine MIE of propane/air at normal atmospheric conditions were performed. However, the ASTM standard does not prescribe any statistical procedure for deriving MIE values from primary test data. We therefore adopted the "highest-possible-border-line" procedure proposed by Moorhouse et al. in 1974. and obtained a MIE of 0.48 mJ, which is very close to the 0.46 rnJ found by these workers, as opposed to the classical Lewis and von Elbe value of only 0.25 mJ. One possible reason for the discrepancy could be the very low ignition probability of only 1% used by Lewis and von Elbe as their MIE criterion. However. when applying both linear and logistic regression analysis to our experimental data, the spark energies yielding 1% probability of ignition were found to be 0.40 +/-0.06 and 0.45 +/- 0.08 mJ, respectively. which are both significantly higher than 0.25 mJ. This may indicate that the classical MIE values for gases and vapours published by Lewis and von Elbe (1961) are perhaps unnecessarily conservative. (C) 2009 Elsevier B.V. All rights reserved.