he relevance of micromixing in fast reactions can be characterized by measuring the product distribution of a known test reaction. Although several test reaction systems have been published, none is suitable for boiling or nearly boiling conditions. This paper presents details of a new test reaction system suitable for sparged aqueous systems at temperatures up to 100degreesC. This system, which is easy to use, provides the basis for experiments carried out in a standard semi-batch stirred tank reactor (STR) agitated with a Chemineer CD-6 hollow-blade impeller. The quantitative characterization provided by liquid micromixing sheds light on the effect of temperature, and hence vapour pressure, in the operation of gas-liquid reactors.