In order to elucidate the unusually strong quadrupole-quadrupole interactions discovered earlier in PrAg, magnetic and neutron diffraction studies are being made of the pseudobinary compounds PrAg1−xCux. For PrAg0.75Cu0.25, which, like PrAg, has a cubic CsCl-type structure, analysis of high-field magnetization data above the antiferromagnetic Néel point (TN∼9 K) shows that the effective biquadratic (quadrupolar) coupling is about twice as strong as in PrAg, whereas the average bilinear exchange coupling is slightly weaker. PrAg0.5Cu0.5, which has a CsCl-type structure at room temperature, was found to transform to an orthorhombic FeB-type structure (similar to that of PrCu) when it is cooled below ∼150 K. Detailed comparison of the two structures of PrAg0.5Cu0.5 indicates that the instability of the CsCl-type structure probably stems from the softening of certain zone-boundary phonons, which presumably grows as x increases towards 0.5. Such phonons would account for strong effective quadrupolar interactions of negative (antiferro) sign, precisely of the type seen in PrAg and PrAg0.75 Cu0.25. a) Work supported by NSF Grant No. DMR 78-12777. b) Work supported by the U.S Department of Energy. c) Work sponsored by the U. S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corp. 1 T. O. Brun, J. S. Kouvel, and G. H. Lander, Phys. Rev. B 13, 5007 (1976).