The trifluorides of La and Gd crystallize in different structures: LaF{sub 3} is of the hexagonal tysonite (P{bar 3}1c) type, while GdF{sub 3} crystallizes in orthorhombic (Pnma) symmetry. Solid solutions of the type La{sub 1 {minus}x}Gd{sub x}F{sub 3} exist in the hexagonal structure to x = 0.5 and convert to the orthorhombic phase at x = 0.75. In this paper, lattice parameters are reported and the x-ray excited luminescence of Gd{sup 3+} in these compounds is discussed with particular emphasis on the Gd{sup 3+} site symmetry difference C{sub 2} in LaF{sub 3} and C{sub s} in GdF{sub 3}.