Single‐crystal samples of phosphorus‐doped Hg0.8Cd0.2 Te were annealed at temperatures varying from 450 to 600 °C in different partial pressures of Hg. Hall effect and mobility measurements were performed on the samples cooled to room temperature. All the samples were found to be p type, with the hole concentration being much less than the total concentration of phosphorus present in the crystals. The hole concentration was found to increase with increase in partial pressure of Hg, in contrast to the behavior observed in undoped crystals. Also, the hole concentration obtained in the doped samples at low Hg pressures was less than that in undoped crystals. The 77‐K hole mobility of the doped samples was similar to that of undoped samples. All these inferences indicate that phosphorus behaves amphoterically in Hg0.8Cd0.2 Te(s), acting as a single acceptor occupying interstitial and tellurium lattice sites at high Hg pressures, and as a single donor occupying Hg lattice sites at low Hg pressures; at intermediate Hg pressures, the majority of the phosphorus appears to be present as electrically neutral pairs formed from the association of the interstitial and substitutional phosphorus species. At low Hg pressures, a large fraction of the phosphorus appears to be present as positively and negatively charged pairs of substitutional phosphorus and vacancies of Hg. Thermodynamic constants evaluated for the incorporation of the various phosphorus species satisfactorily explain the experimental results.