Simultaneous measurements of the O+(2D-2P) emission at 7319-30¿ and the atomic oxygen density in the optically thin region of the atmosphere offer a straightforward means of determining the photoionization frequency, I∞, for the production of O+(2P) ions. The ionization frequency is found to be 4.7¿10-8 sec-1¿30% on June 8, 1974 and 3.4¿10-8 sec-1¿30% on December 17, 1975. The wavelength dependent solar flux intensity in the relevant spectral region was also measured June 8, 1974 on the same spacecraft (Atmosphere Explorer-C). A calculation of I∞ using this solar flux and currently accepted values of the photoionization excitation cross sections gives a value of 4.9¿10-8 sec-1, thus confirming the measured solar EUV flux.