The structural stability of Rb 2 Zn(Cl : -^Br^ (x = 1% and 3%) is investigated using the 35 C1 quadrupole resonance frequency and spin lattice relaxation time (T\q) in the paraelectric (PE) phase. The PE to incommensurate (IC) phase transition temperature 7", observed at 302 K in the pure compound, is lowered to 300 K for x = 1 % and to 293 K for x = 3%. These effects are smaller than in the earlier reported case of 3% Cs substitution in Rb 2 ZnCl 4 , which reduces T x by 25 K. While bigger cation substitution affects only the barrier for anion dynamics about directions perpendicular to the a-axis (direction of low temperature IC modulation wave), bigger anion substitution is found to affect the barrier for anion dynamics about the a-axis as well. The present study also indicates that the cusp like dip in 71 q observed while approaching T x from above (characterizing the soft mode condensation associated with the structural phase transition) is essentially unaffected by anion impurity substitution, in contrast to the case where bigger cation substitution tends to smear the transitional effects. These results seem to suggest the dominant role played by cations in stabilizing the PE phase of these A 2 BX 4 systems, in comparison to anions.