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A simple empirical relation connects the critical temperature of the order-disorder transition T c to the dipolar reorientation rate T' 1 (T) of CN" molecules by the concept of a critical reorientation rate T C ' 1 (T C ).This relation is derived from experimental data on four pure alkali cyanides, the dipole-diluted (KBr)^ :(KCN) X system, and the mixed alkali cyanides (KCN) 1 _ X :(NaCN) x and (RbCN)^ :(KCN) X . The observed disappearance of the order-disorder transition at a certain x value occurs in all mixed systems, when the spread of relaxation rates exceeds a particular value.PACS numbers: 64.70. Kb, 77.40.+i, The study of the reorientation and ordering behavior of diatomic molecules in ionic solids is a field of intense current interest. CN" molecular ions have become phototypical model cases in these studies, because they can be incorporated -in connection with alkali and halide ions-into a variety of regular and irregular solid-state structures. The case of a regular CN" sublattice -the pure alkali cyanides-is characterized at high temperatures by an orientationaily disordered structure, which transforms by a first-order transition at T c into a state of parallel (ferroelastic) order. 1 Dilution of the CrT dipole concentration by halide substitution-in systems such as (KCN) X iCKBR)^ shifts T c gradually to lower temperatures, until at a critical CN" concentration x c ail measurable indications of a longrange order disappear abruptly (observed first in mixed KC1: KCN). 2 It has been speculated that systems with x