In this work we compare the O/N2 number density ratios determined from mass spectrometer measurements to O/N2 ratios determined from photoelectron spectra. The photoelectron spectra analyzed are from the Photoelectron Spectrometer (PES) experiment on the Atmosphere Explorer E (AE-E) satellite. Relative production rates from the photoionization of O and N2 (as calculated from PES data) were used to derive O/N2 ratios for altitudes from 150 to 250 km for a number of orbits of AE-E. In order to demonstrate the validity of the method we have used to analyze the spectra, we include a comparison of a modeled spectrum to a spectrum from PES. The ambient O/N2 is derived for each 15-s interval of the orbits studied. The O/N2 ratio derived from the photoelectron spectrum is compared to the O/N2 ratio from each of two mass spectrometers (Neutral Atmosphere Composition Experiment and open source neutral mass spectrometer) that flew with PES on AE-E. We conclude that atmospheric O/N2 ratios from photoelectron spectra agree (within the error of our measurement) with those from the mass spectrometers. We include in this work suggestions for improvements to the PES experiment that would reduce the uncertainty of any future determinations of atmospheric O/N2 ratios from photoelectron spectra.¿ 1997 American Geophysical Union