Among the products of reduction of 4-(or 5-)nitroglyoxaline, formation of a blue pigment has been observed by various workers. The authors regard this pigment as a diglyoxalone containing the characteristic indigoid group, —CO∙C=C∙CO—. Its formation is dependent on the existence of 4-(or 5-) hydroxyglyoxaline, formed by deamination of the corresponding amino- compound. The hydroxyglyoxaline readily tautomerizes to the glyoxalone, which on oxidation condenses to form the pigment in question.