Hexamine has been mainly employed as a standard substance for the formaldehyde determination by the acetylacetone (AA) and chromotropic acid (CA) method, and formalin by the 4-amino-3-hydrazino-5-mercapto-1, 2, 4-triazole (AHMT) method in"Standard Method of Analysis for Hygienic Chemists"authorized by the Pharmaceutical Society of Japan. Formaldehyde in the test solution prepared by steam distillation of formalin was determined by these three methods, and the data obtained were not coincident with each other. Formalin was employed for the standard substance instead of hexamine in the AA and CA method, so there were no any difference in the data. Formalin should be utilized as the standard substance for the determination of formaldehyde in the AA and CA method just as in the AHMT method.