The derivation and the accuracy of an expression relating the fractional concentration of H+, α1=n(H+)/ne, to the crossover frequency ω12for a VLF proton whistler are discussed. To an accuracy of ±3½% it is found that α1= (264/255) (1 ‐ Λ122), where Λ12= ω12/Ω1and Ω1is the proton gyrofrequency. Values of α1have been deduced from measurement of proton whistler spectrograms for some of the satellite Injun 3 VLF data and plotted against altitude (400–2600 km) and against invariant latitude (20°–63°) for summer daytime and winter nighttime of 1963. The fractional concentration of H+is found to be higher for winter nighttime than for summer daytime at all altitudes and latitudes; the ratio was approximately 3:1 at 1000 km. At 1000 km the value of α1dropped from 0.43 in the 20°–30° invariant latitude range to 0.27 in the 40°–45° range for summer daytime. For winter nighttime α1was nearly constant at 0.82 for 1000 km from 30° to 50° latitude, but dropped to 0.75 in the 50°–63° latitude range. Near 2400 km for summer daytime, α1drops from 0.65 at 46° latitude to 0.20 at 56° latitude. This same tendency is observed for winter nighttime, apparently being due to auroral‐zone heating. These observations are consistent with the assumption that the heavier ions tend to predominate with increasing latitude for a given altitude. Comparison of these VLF proton whistler results for α1is made with reasonably good agreement to rocket ion mass spectrometer results of NASA 8.23 and to Alouette 1 VLF results deduced from lower hybrid resonance frequency. This general agreement establishes the VLF radio technique as an independent method for determining ion concentrations in the ionosphere. The general equations for uniquely determining the concentration of O+, He+, and H+from observation of the critical frequencies (crossovers, hybrid resonances, and cutoffs) due to the presence of ions are