Ternary samples with the composition (R E_{3} \mathrm{Ag}{x} \mathrm{Ga}{11-r}, R E=\mathrm{Y}, \mathrm{Gd}, \mathrm{Tb}, \mathrm{Dy}, \mathrm{Ho}, \mathrm{Er}, \mathrm{Tm}), and (\mathrm{Yb}), have been synthesized from the elements by arc- or (\mathrm{HF}-) melting and annealing at (600^{\circ} \mathrm{C}). In all cases isotypy with the (\mathrm{La}{3} \mathrm{Al}{11})-type has been confirmed from X-ray powder data. The homogeneity region of the (\mathrm{Yb}{3} \mathrm{Ag}{x} \mathrm{Ga}{11-x}) phase has been established by (\mathrm{X})-ray powder diffraction analysis and ranges at (600^{\circ} \mathrm{C}) from (x=2.6) to (x=3.8). Unit cell parameters in this region reveal a remarkable positive deviation from Vegard's rule corresponding with a partial composition-dependent atomic ordering in the various sites of the (\mathrm{La}{3} \mathrm{Al}{11})-type structure. The occupation mode in the (\mathrm{Yb})-containing alloys has been studied in detail by full profile X-ray powder analysis for compositions (x=2.55) and 3.65 and by X-ray single crystal counter data refinement for the alloy with (x=3.0), confirming in all cases full consistency with the (\mathrm{La}{3} \mathrm{Al}{1}-)-type structure with the space group Immm. Throughout the homogeneous range (\mathrm{Ag}) was observed to preferentially occupy the ( (2 \mathrm{c}) ) sites. The relative increase of the (\mathrm{Ag}) occupancy in (2c) with increasing content of silver, however, decreases and for compositions (x>3.0, \mathrm{Ag}) was found to enter the (81) positions. In correspondence with magnetic susceptibility measurements the peculiar lattice parameter variation observed is due to this complicated mode of (\mathrm{Ga} / \mathrm{Ag})-substitution rather than to a valence change on the ytterbium atom. Magnetic susceptibilities were determined over a temperature range from 4 to (550 \mathrm{~K}). Above LNT, magnetic behavior of (\mathrm{Gd}(x=2.8), \mathrm{Tb}, \mathrm{Dy}), Ho, Er, and Tm compounds ( (x=3.8) ) corresponds to the paramagnetism of ideal tripositive rare earth elements. Below (T=25 \mathrm{~K}) antiferromagnetic ordering of the (R E)-moments is encountered for the (\mathrm{Yb}{3} \mathrm{Ag}{x} \mathrm{Ga}{11-x}) alloys. The (\mathrm{Yb})-atom was found to be in a nonmagnetic (\mathrm{Yb}^{2+}) ground state throughout the entire homogeneous range. The susceptibilities are practically temperature independent paramagnetic down to (20 \mathrm{~K} . \mathrm{Y}{3} \mathrm{Ag}{3} \mathrm{Ga}_{8}) is a diamagnet. 1993 Academic Press, lnc.