The effects of reactor arrangement, catalyst particle size and void volume on the oxidative dehydrogenation (ODH) of propane and iso-butane are studied. In an empty quartz reactor, a high conversion of alkane (up to 36% with a 40% ODH selectivity) can be achieved. Packing of the reactor with different solids (quartz, gamma-Al2O3, complex V-containing oxide catalysts) leads to a drastic change in reaction parameters, which is strongly dependent on the chemical nature of solids and their particle size. The analysis of observed phenomena indicates that: (a) the termination of gas-phase chain reaction takes place on the surface of any solid material; (b) certain solids behave as active generators of free radicals, thus initiating the new reaction pathway which develops both in the gas-phase and on the solid surface.