A recently developed method to characterize the band gap energies of III-V optosemiconductors was utilized to determine temperature-invariant band gap features of multijunction solar cells. The method is based on measuring electroluminescent spectra of the solar cells at different temperatures. The normalized spectra reveal temperature-invariant energy values of the different junctions which are further converted to band gap energies. The method utilization requires a calibrated spectroradiometer and a temperature controlled mounting base for the solar cell under test, however, no knowledge about the absolute temperature of the cell under measurement. The method was tested on GaAs and GaInP solar cells that consist of single and dual junctions. The band gap energies were also derived from spectral response measurements. The differences of the determined band gap energies from the literature values were smaller than 1.1%. Compared with other band gap determination methods, the developed method yields temperature-invariant band gap characteristics; with a known uncertainty, that separated the different junctions in a multijunction device without individual biasing for the different junctions. In addition, a temperature-independent characterization parameter ensures that the operating conditions of the device under test do not affect the results.