A novel composite superionic system, [Cu₂HgI₄ : 0.𝑥AgI], (𝑥 = 0.2, 0.4, 0.6 mol wt.%), was prepared. A [Cu₂HgI₄] system was used as the host. Electrical conductivity was measured to study the transition behaviour at frequencies of 100 Hz, 120 Hz, 1 kHz and 10 kHz in the temperature range 90–170°C using a Gen Rad 1659 RLC Digibridge. Conductivity increased sharply during the 𝛽–𝛼 phase transition. Upon increasing the dopant-to-host ratio, the conductivity of the superionic system exhibited Arrhenius (thermally activated)-type behaviour. DTA, DTG, TGA and X-ray powder diffraction were performed to confirm doping effect and transition in the host. The phase transition temperature increased with an increase in the dopant concentration. Activation energies in eV for pre- and post-transition phase behaviour are also reported. Due to an interaction between [Cu₂HgI₄] and AgI, the addition of AgI to [Cu₂HgI₄] shifted the phase transition of the host [Cu₂HgI₄].