Azo pigments are typical of the classical pigments as characterized by the azo group (−N=N−) and used widely in painting and imaging industries. In azo pigments, however, there is still a pending problem associated with the azo or hydrazone structure in the solid state. As a step to clarify this, structure analysis of methyl orange (MO: the simplest azo compound) has been carried out together with its electronic structure. MO is found to crystallize in the space group of P2₁2₁2₁ which includes three molecules in the asymmetric unit. The three N/N bonds are typical of the azo structure and the three Na atoms form two kinds of complexes: two seven-coordinate and one five-coordinate complexes. These are connected through the sulfonic anions and form one dimensional polymer along the a-axis. Furthermore, solvated MO is found to give an additional band around 580 nm on going from solution to the solid state. This makes the color orange.