Abstract The reaction between thiocarbamate herbicides and 2,6-dichlorobenzoquinone- N-chloroimine or 2,6-dibromobenzoquinone- N-chloroimine is suitable for the detection of these herbicides on thin-layer plates with high sensitivity. The reactions were followed by infrared, nuclear magnetic resonance and mass spectrometry. We have established the formation of 2,6-dichlorobenzoquinone- S-alkyl sulphenylimines. In the case of the bromo-derivative, halogen exchange and substitution on the quinone ring took place simultaneously leading to the formation of mixed halogenated 2,6-dihalo- and, in addition, 2,3,6-trihalobenzoquinone- S-alkyl sulphenylimines. The final product of the detection reaction, i.e. 2,6-dichlorobenzoquinone- S-alkyl sulphenylimine was reacted with 2,6-dibromobenzoquinone- N-chloroimine where 2,6-dichloro-3-bromobenzoquinone- S-alkyl sulphenylimine formed as a consequence of the looser bromine—carbon linkage on the 2 and 6 positions of the quinone ring.