Two simple mercapto thiadiazole-based sensors have been synthesised by a convenient method and exhibit excellent sensitivity and selectivity for Hg2+ in DMSO/H2O (1:1, v/v) aqueous solution. The sensors react with Hg2+ to form stable complexes and the association constants, Ka, are 4.4 × 104 M−1 and 1.0 × 103 M−1, respectively. Furthermore, the detection limit of one sensor towards Hg2+ is 8.0 × 10−7 M.