We present the electrolyte-clectroreflectnnce spectrum of In0.75Ga0.25As0.56P0.44 between 1.2 and 3.1 eV at room temperature. Analysis of the spectrum has enabled us to determine the values of the band gap and spin-orbit splitting. These results are compared with the calculations from interpolations of the accepted values for the related binary compounds and measurements made on other In GaAsP quaternary alloy.