This paper discusses the construction of a cylinder flow reactor used for studying heterogeneous reactions, with particular reference to reactions involving NOx. This system incorporates methods of gas analysis such as optical absorption (NO2), chemiluminescent (NO or O3), gas chromatography (N2or O2) and mass spectrometry (SO2, etc). The system was constructed such that experimental conditions could be varied over a large pressure range from 1 to 700 Torr with a relative humidity variation of 0–90%.