We have measured Schottky‐barrier electroreflectance spectra of an In0.79Ga0.21As0.54P0.46LPE layer at 81 K. Analysis of the spectra has enabled us to determine the interband reduced mass of the quaternary alloy together with the values of the band gap and spin‐orbit splitting. The obtained reduced mass &mgr;=0.033m0, in agreement with the value calculated from interpolations of the accepted values for the related binary compounds.