Several useful concepts derived from density functional theory have been applied to the study of the chemical reactivity of the more stable forms of the isomeric thiadiazolines. Dipole moments, total hardness of the molecules in terms of the calculated ionization potentials and electron affinities as a measure of aromaticity and the condensed Fukui functions related to the variation of the net charges of the atoms resulting from a Mulliken population analysis were calculated in order to determine the reactivity of different sites within the molecules studied. The net charges have been obtained from calculations made in the context of the Hartree-Fock and of local (SVWN) and nonlocal (BLYP, B3LYP) density functional approximations. The results were compared with the existing experimental evidence on thiadiazolines and related compounds. Electronic properties calculated using the hybrid B3LYP proved to be the most accurate. It is concluded that the 1, 2, 3-thiadiazoline molecule would be the best starting monomer of the four isomers to be considered for a polymerization reaction.