Thiazine-indigos are novel hydrogen-bonded, yellow to red pigments currently developed by Clariant. Commercially available chlorinated derivative (THI), for example, exhibits vivid red in the solid state while only pale yellow in solution. Structure analysis of THI has therefore been carried out in order to elucidate the color generation mechanism on going from solution to the solid state. THI is found to crystallize in space group P–1 with Z = 1. The molecule is entirely planar characterized by Ci symmetry. There are chains of intermolecular N–H…O hydrogen bonds on the molecular plane. The present intermolecular hydrogen bonds hold the molecules firmly together, leading to a high light and heat stability of THI as well as low solubility in solvents. The color change from yellow to red upon crystallization has been interpreted as arising mainly from intermolecular interactions between transition dipoles.