AbstractThe use of pure benzole as a raw material for the manafacture of plastics has demanded the production of a grade containing very low quantities of sulphur as thiophen. Recent experience has shown that, in order to estimate this thiophen, which may be present in quantities of the order of 0·001% wt. sulphur as thiophen, reliance has been placed almost entirely on the well‐known isatin test, although many variations of the test are being used and comparable results are not being obtained. The present work was undertaken to enable a reliable test to be suggested that is capable of standardisation in the manner of other tests commonly used for testing tar and its produc