Photodecomposition of pyridoxine (PN) in an aqueous solution was accelerated by the addition of FMN, whereas PN inhibited that of FMN. The accelerated photodecomposition of PN was apparently pseudo-first order, and was most rapid at pH 7.0, but became markedly slower especially with decreasing pH of the buffer solution. The decomposition could not be prevented even in a brown ampule without complete shielding with aluminium foil. Effect of some drugs further added to the reaction mixture was examined, and tyrosine and tryptophan were found to inhibit the decomposition. The photodecomposition of PN in infusion solutions, to which commercially available injections of PN and FMN were added, was studied. The decomposition was marked in sugar and saline infusion solution, although it was much less in amino acids infusion solutions. The results indicate that infusion solutions should be protected from light when they contain both PN and FMN.