Carrier species in arsenic-implanted p– and n–type Hg0.7Cd0.3Te films grown by molecular-beam epitaxy were investigated with the use of the Hall-effect studies and mobility spectrum analysis. The implantation was performed with ion energy 190 or 350 keV and ion fluence ranging from 1012 to 1015 cm2. A substantial difference between carrier species in Hg0.7Cd0.3Te and Hg0.8Cd0.2Te films subjected to arsenic implantation and post- implantation activation annealing was established. In particular, arsenic implantation in p–type Hg0.7Cd0.3Te in most cases lead to the formation of n+–p– (not n+–n–p–) structures, and in n–type Hg0.7Cd0.3Te films post- implantation activation annealing lead to modification of the electrical parameters of the n–type ‘base’, in contrast to Hg0.8Cd0.2Te material studied earlier. The difference in carrier species formed in arsenic-implanted Hg0.7Cd0.3Te and Hg0.8Cd0.2Te films was tentatively explained by different background impurity concentrations in the films with different chemical composition.