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Abstract The state with effective total moment J eff = 1/2 stabilized by the spin-orbit coupling is known to suppress Jahn-Teller distortions and may induce a strong exchange anisotropy. This in turn may lead to the formation of an elusive spin-liquid state in real materials. While recent studies have demonstrated that such a situation can be realized in 3d transition-metal compounds such as those based on Co2+ and Cu2+, diagnosis of J eff = 1/2 state remains challenging. We show that resonant inelastic X-ray scattering is an effective tool to probe this state and apply it to CuAl2O4, material where Cu2+ ions were previously proposed to be in the J eff = 1/2 state. Our results unambiguously demonstrate that, contrary to previous expectations, a competitive (to J eff = 1/2) Jahn-Teller state realizes in this compound.