The heat capacity of the mixed crystals NH
4
Br
0.20
CI
0.80
and NH
4
Br
0.74
CI
0.26
has been measured from
ca
. 8 to 300 K. The first of these solids has, as expected, two gradual transitions, the lower having an entropy of transition ∆
S
λ
of 3.79 J K
–1
mol
–1
, and the upper a ∆
S
λ
of 4.51 J K
–1
mol
–1
. The heat capacity between these two transitions is always considerably above the ‘normal’ or baseline value, and it appears that the orientational disordering of the ammonium ions, while completed in the
γ
→ β transition, commences in the δ-phase. The other, bromide-rich mixed crystal shows no sign of a δ →
γ
transition, but has a
λ
-type
γ
→ β transition which culminates at 262.5 K, with ∆
S
λ
= 7.31 J K
–1
mol
–1
. The heat capacity of both mixed crystals in the β-phase, like that of the pure components, is ‘anomalously’ high, the additional heat capacity for the solid solutions being larger than that for the pure salts. The phase diagram for the ammonium-chloride–ammonium-bromide system is shown; it combines previous information with that provided by the heat-capacity results reported in this and previous papers.