By colorimetry of hydroxyurea and hydroxylamine with 8-hydroxyquinoline, it was found that hydroxylamine was formed from hydroxyurea by heating. The formation was enhanced at higher temperature and by addition of hydrochloric acid. On the other hand, hydroxylamine as well as hydroxyurea was remarkably decreased by heating in alkaline media. Coloration of several substances with the same reagent was also examined. Substances examined were urea, methyl alcohol, ethyl alcohol, ammonium hydroxide, sodium hydroxide, potassium hydroxide, phenylhydrazine, uric acid, phenylthiourea and isonicotinic acid hydrazide besides hydroxylamine and hydroxyurea. All they had no effect on colorimetry of hydroxylamine, while hydroxylamine and phenylhydrazine disturbed the estimation of hydroxyurea.