The crystal structures of isomeric rac-2-(4-nitrophenyl)-3-phenyl-2,3,5,6-tetrahydro-4H-1,3-thiazin-4-one (C16H14N2O3S) (1) and (2S)-2-(3-nitrophenyl)-3-phenyl-2,3,5,6-tetrahydro-4H-1,3-thiazin-4-one (C16H14N2O3S) (2) are reported here. While 1 crystallizes in a centrosymmetric space group, the crystal of 2 chosen for data collection has molecules only with (2S) chirality. This is the result of spontaneous resolution during crystallization, as the synthesis produces a racemic mixture. A crystal with (2R) molecules was also found in the same crystallization vial (structure factors available). The six-membered thiazine ring in both 1 and 2 displays an envelope conformation with the S atom forming the flap. The aryl rings in both structures adopt an approximate V shape with angles between their planes of 46.97 (14)° in 1 and 58.37 (10)° in 2. In both structures, the molecules form layers in the ab plane. Within such a layer in 1, one of the O atoms of the nitrophenyl group accepts a C—H...O hydrogen bond from the CH group at position 5 of the thiazine ring of a molecule of opposite chirality, forming chains along the a-axis direction. Each of the thiazine rings also participate in C—H...O bonds with the same carbon atom as above, resulting in chains along the b-axis direction, albeit of monochiral type. Adjacent layers are consolidated along the c-axis direction by pairs of parallel hydrogen bonds (C—H...O type) between the nitrophenyl groups of enantiomers. In 2, the two C—H...O hydrogen bonds contribute to chain formation along the b-axis direction. Weak edge-to-face interactions between the aryl groups of neighbouring molecules in 1, and C—H...π interactions between a thiazine ring CH group and a phenyl group of a neighboring molecule in 2 are also observed.