A survey study on electrophilic chlorination of aromatics with trichloroisocyanuric acid (TCICA) in Brønsted-acidic imidazolium ionic liquid [BMIM(SO₃H)][OTf] is reported. The reactions are performed under very mild conditions (at ~50°C) and give good to excellent yields, depending on the substrates. Chemoselectivity for mono- v. dichlorination can be tuned by changing the arene-to-TCICA ratio and the reaction time. The survey study and competitive experiments suggest that triprotonated/protosolvated TCICA is a selective/moderately reactive transfer-chlorination electrophile. Density functional theory was used as guide to obtain further insight into the nature of the chlorination electrophile and the transfer-chlorination step.