By means of x-ray absorption spectroscopy and model Hartree-Fock calculations, we have investigated the electronic structure of layered t 2g electron systems such as Ca2 - xSrxRuO 4 and Bi2Sr2Co2O9. For the hole-doped Co-O triangular lattice in Bi2Sr2Co 2O9, the x-ray absorption spectral line shape indicates that the carriers in the t2g band mainly have the a1g symmetry. This result is supported by model Hartee-Fock calculations on the CoO2 lattice model. In Ca2 - xSrxRuO 4, the interplay between the orbital ordering and the Mott transition is demonstrated by the x-ray absorption measurement. It is shown that the orbital ordering accompanied by the lattice distortion is important in describing the insulating state of Ca2 - xSrxRuO 4. This idea is also supported by model Hartee-Fock calculations on the RuO2 lattice model.
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