An experimental study of cadmium substitution into AgXS₂ phases (X = As, Bi, Sb) indicates significant amounts of cadmium can be incorporated in the structure by the mechanism 2Cd ⇌ Ag + X. The limit of substitution of Cd in the high-temperature polymorph of miargyrite, β-AgSbS₂ is 6.2 at.%, whereas the low-temperature polymorph, α-AgSbS₂, can accommodate at least 12 at.% cadmium. Substitution of Cd into the cubic β-AgSbS₂ induces a small monoclinic distortion and the unit cell becomes pseudo-cubic. The α ⇌ β transition in Cd-substituted miargyrites is modified by the solute ions, with both a reduction of T_c and transition smearing evident. Similar effects are also recorded in Cd-substituted AgAsS₂ and AgBiS₂.