We have performed hard x-ray photoemission spectroscopy (HAXPES) for Yb-based Kondo lattice compounds; an antiferromagnetic heavy-fermion system YbNi 3 Al 9 and a valence fluctuation system YbNi 3 Ga 9 . The Yb 3d 5/2 spectra of YbNi 3 Ga 9 showed both Yb 2+ and Yb 3+ -derived structures indicating strong valence fluctuation, and the intensity of Yb 2+ (Yb 3+ ) structures gradually increased (decreased) on cooling. The Yb 3d 5/2 spectra of YbNi 3 Al 9 mostly consisted of Yb 3+ -derived

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... s and showed little temperature dependence. The Yb valences of YbNi 3 Ga 9 and YbNi 3 Al 9 at 22 K were evaluated to be 2.43 and 2.97, respectively. Based on the results of the Ni 2p and valence-band HAXPES spectra together with soft x-ray valence-band spectra, we described that the difference of physical properties of YbNi 3 X 9 (X = Al, Ga) is derived from the differences of the 4f -hole level relative to the Fermi level (E F ) and Ni 3d density of states at E F . The HAXPES results on the Yb valences were consistent with those obtained by x-ray absorption spectroscopy using the partial fluorescence yield mode and resonant x-ray emission spectroscopy at the Yb L 3 edge.