The reaction products of cyanide with pyridoxal and with pyridoxal 5′-phosphate were reexamined. Under weak alkaline conditions employed in the fluorometric assay of these compounds, they were converted almost quantitatively to a product having strong fluorescence. The results of ultraviolet, infrared, and fluorescence spectrophotometry, paper chromatography, paper electrophoresis, and other tests confirm the identity of the reaction products as 4-pyridoxic acid lactone and 4-pyridoxic acid 5′-phosphate, respectively. © 1968.