Selective anion sensors can be used in the colourimetric detection of certain anions. In the present study, two new 1,2,4-triazole derivatives containing thiol group and azo moiety 3a (nitrile derivative) and 3b (methoxy derivative) were synthesised by condensation of 4-(4-amino-5-mercapto-4 H-1,2,4-triazol-3-yl)phenol (1) and aromatic aldehydes containing azo groups 2a and 2b under acidic conditions. The nitrile derivative 3a acted as a selective naked-eye sensor for acetate anions in a mixture of dimethyl sulphoxide and water (9:1 by volume). It interacted with the acetate anions in a 1:2 stoichiometry with strong positive cooperativity and an overall association constant (β12) exceeding 2 × 108 M−2. The detection limit of 3a towards the acetate anion was found to be 0.66 × 10−5 M. The DFT studies showed that both 3a and 3b were similar in their electronic properties, but the internal charge transfer for 3a was responsible for the positive response to acetate anions.