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 moiety3a(nitrile derivative) and3b(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 groups2aand2bunder acidic conditions. The nitrile derivative3aacted 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 (beta(12)) exceeding 2 x 10(8)M(-2). The detection limit of3atowards the acetate anion was found to be 0.66 x 10(-5)M. The DFT studies showed that both3aand3bwere similar in their electronic properties, but the internal charge transfer for3awas responsible for the positive response to acetate anions.