Abstract: The luminescence of Ce³⁺ in LnOX was studied by excitation and emission spectroscopy.The LnOX studied belong to the same structure type, tetragonal, with space group D_4h⁷-P₄/nmm, site symmetry of Ln³⁺ C_4v. The spectra show that when X is kept identical but with different Ln the spectra only manifest minor differences: whereas with different X there are rather noticeable changes in spectra, the corresponding peaks in the spectra shift to lower energy in the order of Cl-Br-I. Some differences were observed in the behavior of La compound in comparing with that of Gd or Y compounds, especially in the excitation spectra. It was discussed in terms of the differences bstween ionic radius of Ce³⁺ and those of the host cations.