Synthesis gas (Syngas) is an intermediate in a variety of industrial processes. Its production is energy and capital intensive and any improvement of existing technologies allowing simpler and economic production is of great interest. Recently, a new method known as short contact time-catalytic partial oxidation (SCT-CPO) has been developed into a commercial technology [1-4]. SCT-CPO is an entirely heterogeneous catalytic process converting premixed flammable feedstocks inside a very small reactor. In order to ensure safety and a high selectivity towards CO and H-2 it has been important to determine and understand flammability properties of the gaseous reactant mixtures. Here we report on the results obtained within a windowed tube reactor equipped with multiple photodetectors and pressure transducers that has allowed the study of ignition, flame propagation, and explosion characteristics of gas mixtures similar to those used as reactants in the SCT-CPO reactor. The tests were conducted at various pressures with different amounts of steam and two different compositions of natural gas (NG). A flammability boundary for each mixture, based on normalized pressure and mole fraction of steam, was determined. The results conclude that these mixtures' flammability could be suppressed in two very different ways. Depending on the adiabatic flame temperature of the mixture, suppression could be caused by steam's chemical influence increasing chain-termination or by a large amount of steam decreasing the reaction zone temperature. (C) 2010 Elsevier B.V. All rights reserved.