The 'isothermal vapor phase epitaxy (iso-VPE) of mercury cadmium telluride on finite cadmium telluride substrate- systems,' consists in the complete transformation of starting CdTe films, on foreign-inert-substrates grown (by MOCVD or MBE), into compositionally controlled and uniform HgCdTe/foreign-substrate structures. The HgTe vapor phase growth and the HgTe/CdTe solid-state interdiffusion are the physical phenomena controlling the iso-VPE process evolution. Working at 530 degrees Celsius, under experimental conditions characterized by HgTe/CdTe interdiffusion rates higher than the HgTe growth rates, the possibility to prepare high quality Hg1-xCdxTe/Sapphire structures has been demonstrated and reported last year. Recently the same process has been successfully experimented on CdTe/Silicon substrates too. For the first time the HgCdTe/Silicon iso-VPE structures characteristics are reported in comparison with the equivalent HgCdTe/Sapphire films. Because of the good solid-state chemical compatibility between Hg1-xCdxTe alloys and silicon, well defined metallurgic interfaces between active layers and silicon substrates have been obtained. Rocking curves with FWHM in the 110 - 130 arcsec range have been measured on 15 micrometer thick HgCdTe/silicon iso-VPE structures. The films as-grown electrical characteristics are p-type, according to a mercury vacancy dominated defectivity.