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The present work aimed to synthesize 2-methylthio-triazoloquinazoline derivatives and study their X-ray, NMR, DFT and Hirshfeld characteristics. The cyclocondensation of dimethyl-<i>N</i>-cyanodithiocarbonate with 2-hydrazinobenzoic acid hydrochloride resulted in an intermediate, 2-methylthio-[1,2,4]triazolo[1,5-<i>a</i>]quinazolin-5-one (<b>A</b>), which upon treatment with phosphorus pentasulfide, transformed into the 2-methylthio-[1,2,4]triazolo[1,5-<i>a</i>]quinazolin-5-thione (<b>B</b>). Reaction of 2-methylthio-triazoloquinazolines (<b>A</b>&<b>B</b>) with alkyl halides (allyl bromide and ethyl iodide) in basic medium afforded 4-allyl-2-methylthio-[1,2,4]triazolo[1,5-<i>a</i>]quinazolin-5-one (<b>1</b>; <i>N</i>-alkylated) and 5-ethylthio-2-methylthio-[1,2,4]triazolo[1,5-<i>a</i>]quinazoline (<b>2</b>; <i>S</i>-alkylated), respectively. Their molecular and supramolecular structures were presented. Unambiguously, the molecular structures of <b>1</b> and <b>2</b> were confirmed via NMR and single-crystal X-ray diffraction. The resulting findings confirmed the structures of <b>1</b> and <b>2</b> and determined their crystalized system (monoclinic system; <i>P</i>2₁/<i>n</i> space group). Hirshfeld analysis of <b>1</b> revealed the importance of the significantly short O···H (6.7%), S···S (1.2%) and C···C (2.8%); however, the short H···H (42.6%), S···H (16.3%) and C···C (4.3%) were showed in <b>2</b> by intermolecular interactions in the molecular packing. The 1,2,4-triazoloquinzolines (<b>1&2</b>) were anticipated to be relatively polar compounds with net dipole moments of 2.9284 and 4.2127 Debye, respectively. The molecular electrostatic potential, atomic charge distribution maps and reactivity descriptors for <b>1</b> and <b>2</b> were also determined. The calculated nuclear magnetic resonance spectra of the targets <b>1</b> and <b>2</b> were well correlated with the experimental data.