Extensive homogeneous gas-phase reactions were observed when decane was used as the hydrocarbon reductant for the selective reduction of NOx. The catalytic performance of a SnO2/CoOx/Al2O3 catalyst was found to be strongly dependent on the extent of the homogeneous reaction in the pre-catalytic volume. The effect of the homogeneous reaction on the catalytic performance also depended on whether SO2 was present in the feed. By filling the pre-catalytic volume with 25-35 mesh irregularly shaped quartz chips, gas-phase reaction was suppressed significantly. This methodology was used to evaluate the inherent catalytic performance of SnO2/CoOx/Al2O3 and SnO2/Al2O3 catalysts with decane as a reductant. It was found that in the absence of SO2, SnO2/Al2O3 was a better catalyst than SnO2/CoOx/Al2O3, but in the presence of 30 ppm of SO2 the latter was a far better catalyst.