The absorption spectra of samples of nitric oxide were investigated for the purpose of obtaining the relative abundance of the nitrogen and oxygen isotopes. Nitrogen from four different sources was investigated: air nitrogen, obtained from the Haber and Birkeland-Eyde processes; nitrogen from Pennsylvania coal and from Chili saltpeter. One sample of oxygen was obtained from a Pre-Cambrian magnetite. A method has been described for converting the nitrogen and oxygen from these different sources into nitric oxide. A method of photographic photometry has been applied to the spectra obtained. The procedure for obtaining the relative abundance of the isotopes is discussed together with the possible sources of error. The average of 32 measurements gives for the ratio of ${\mathrm{N}}^{15}$${\mathrm{O}}^{16}$/${\mathrm{N}}^{14}$${\mathrm{O}}^{18}$ the result 0.549±0.007:1. This gives the value 346:1 for the ratio ${\mathrm{N}}^{14}$/${\mathrm{N}}^{15}$. The isotopic composition of the four samples of nitrogen and two samples of oxygen is constant within approximately 10 percent.

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