The Bureau of Mines determined the composition of the vapors liberated during thermal decomposition of aluminium nitrate nonahydrate. Small samples were heated from room temperature to 550 deg. C with initial heating rates from 50 deg to 200 deg c/min. Gases were rapidly swept away from the heated chamber with ar and subjected to immediate analysis in order to minimize reaction between the components. In addition to h2o, four n-containing compounds were identified: hno3, NO2, no, and n2o. N2 could not be determined in the presence of the nitrogen oxides, and attempted separation from the oxides led to the formation of more nitrogen. Relative amounts of each component were found to vary over a period of time; however, total amounts measured with different heating rates were reasonably constant and were typically as follows: 84.2 pct hno3, 7.1 pct NO2, 4.3 pct no, and 4.2 pct n2o. Hno3 and no were determined from chemical reactions with a permanganate solution, n20 was measured with a mass spectrometer, and NO2 was measured with a spectrophotometer. Details of the methods used are described. Since NO2 and no can readily be converted to hno3, about 96 pct of the n from the decomposition can be recovered as hno3.