A method and its corresponding first results are presented for the determination of maximum safe capillary diameters, which may be used to describe the extended safety range of operation conditions for partial gas phase oxidation reactions in microreactor devices. Sections of stainless steel microcapillaries of different inner diameters were mounted between a primary and a secondary chamber. A detonation was ignited in the primary chamber and the propagation of the detonation through the stainless steel microcapillaries was monitored by pressure transducers located between the sections of the microcapillaries. This set-up was used in order to determine explosion velocities inside the capillaries, maximum safe initial pressures, and corresponding maximum safe capillary diameters. Initial investigations were performed with an ideal stoichiometric mixture necessary for complete combustion of ethene with oxygen at room temperature. The measured maximum safe capillary diameters obey an indirect proportionality to the initial pressures. The maximum safe capillary diameter can be estimated on the basis of the lambda/3-rule. (C) 2009 Elsevier Ltd. All rights reserved.