The solid-state linkage isomerization [Co(NH3)5NO2]Br2 double-line arrow pointing left and right [Co(NH3)5ONO]Br2 was studied, at room temperature, using the X-ray powder diffraction technique. The structure of the photochemically synthesized [Co(NH3)5-ONO]Br2 was solved in the orthorbombic space group Cmcm, with a = 10.8011(7) angstrom, b = 8.9692(6) angstrom, c = 10.6090(6) angstrom, and Z = 4, and refined by the Rietveld method, down to R(profile) = 0.119, for 4000 data points collected in the 10-90-degrees (2theta) range. The changes in the lattice parameters and the structural evolution in the course of the ONO double-line arrow pointing right NO2 linkage isomerization showed the intramolecular nature of the isomerization, with the powders remaining a monophase system at any time.