We have examined the potential of Lg4 emission spectroscopy as a new method for valence determination. Sm Lg4 emission spectra of a mixed valence compound, SmB6 , as well as Sm standard references were recorded using a multicrystal, multidetector spectrometer at 298 K. The Lg4 spectra exhibited a large chemical shift (~4.5 eV) and substantial profile changes depending on the Sm oxidation state. Splitting of the Sm Lg 4 band was attributed to spin-dependent atomic multiplets, and valence-dependent edge-shifts of the Sm L I XANES spectra strongly suggest that the chemical shift of the Lg4 emission is mainly due to the screening effects of the Sm L I core. The Lg4 spectrum of SmB6 was well reproduced by the averaged spectra of SmS and SmF3 , indicating a SmB6 valence of 2.65 (0.03). The advantages of the Lg4 method, including simple data treatment and no need for an excitation energy scan, will encourage its use in several areas of materials science.