Four bicyclic 4H-[1,2,4]thiadiazines 1a-d were prepared in 74-88% yields in two steps from the corresponding amidines 2. Three of them, 1a, 1b, and 1d, were obtained by thermal elimination of propene from the intermediate S-propylsulfilimines 12. The pyrazino derivative 1c was formed upon thermolysis of sulfoxide 14c obtained from 2c. The Ei mechanism was investigated using DFT methods. The elimination in the sulfilimine appears to be more favorable by about 2 kcal/mol than in the analogous sulfoxide. Crystal and molecular structures of three out of the four thiadiazines were established by single-crystal X-ray analysis. All thiadiazines were found as the 4H tautomers with the heterocyclic ring puckered along the S(1)⋯N(4) line. The benzo derivative 1a forms a unidimensional N(4)-H⋯N(2) chain, the pyrazino derivative 1c forms dimeric pairs with two synergistic hydrogen bonds, and the crystal structure of 1d is characterized by strong C6F 4⋯C6H5 quadrupolar interactions.